summaryrefslogtreecommitdiffstats
path: root/Documentation/DocBook/kernel-locking.tmpl
blob: 67e7ab41c0a6592a747bd414e3bf5923f111ca6a (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

<book id="LKLockingGuide">
 <bookinfo>
  <title>Unreliable Guide To Locking</title>
  
  <authorgroup>
   <author>
    <firstname>Rusty</firstname>
    <surname>Russell</surname>
    <affiliation>
     <address>
      <email>rusty@rustcorp.com.au</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <copyright>
   <year>2003</year>
   <holder>Rusty Russell</holder>
  </copyright>

  <legalnotice>
   <para>
     This documentation is free software; you can redistribute
     it and/or modify it under the terms of the GNU General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later
     version.
   </para>
      
   <para>
     This program is distributed in the hope that it will be
     useful, but WITHOUT ANY WARRANTY; without even the implied
     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
     See the GNU General Public License for more details.
   </para>
      
   <para>
     You should have received a copy of the GNU General Public
     License along with this program; if not, write to the Free
     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
     MA 02111-1307 USA
   </para>
      
   <para>
     For more details see the file COPYING in the source
     distribution of Linux.
   </para>
  </legalnotice>
 </bookinfo>

 <toc></toc>
  <chapter id="intro">
   <title>Introduction</title>
   <para>
     Welcome, to Rusty's Remarkably Unreliable Guide to Kernel
     Locking issues.  This document describes the locking systems in
     the Linux Kernel in 2.6.
   </para>
   <para>
     With the wide availability of HyperThreading, and <firstterm
     linkend="gloss-preemption">preemption </firstterm> in the Linux
     Kernel, everyone hacking on the kernel needs to know the
     fundamentals of concurrency and locking for
     <firstterm linkend="gloss-smp"><acronym>SMP</acronym></firstterm>.
   </para>
  </chapter>

   <chapter id="races">
    <title>The Problem With Concurrency</title>
    <para>
      (Skip this if you know what a Race Condition is).
    </para>
    <para>
      In a normal program, you can increment a counter like so:
    </para>
    <programlisting>
      very_important_count++;
    </programlisting>

    <para>
      This is what they would expect to happen:
    </para>

    <table>
     <title>Expected Results</title>

     <tgroup cols="2" align="left">

      <thead>
       <row>
        <entry>Instance 1</entry>
        <entry>Instance 2</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>read very_important_count (5)</entry>
        <entry></entry>
       </row>
       <row>
        <entry>add 1 (6)</entry>
        <entry></entry>
       </row>
       <row>
        <entry>write very_important_count (6)</entry>
        <entry></entry>
       </row>
       <row>
        <entry></entry>
        <entry>read very_important_count (6)</entry>
       </row>
       <row>
        <entry></entry>
        <entry>add 1 (7)</entry>
       </row>
       <row>
        <entry></entry>
        <entry>write very_important_count (7)</entry>
       </row>
      </tbody>

     </tgroup>
    </table>

    <para>
     This is what might happen:
    </para>

    <table>
     <title>Possible Results</title>

     <tgroup cols="2" align="left">
      <thead>
       <row>
        <entry>Instance 1</entry>
        <entry>Instance 2</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>read very_important_count (5)</entry>
        <entry></entry>
       </row>
       <row>
        <entry></entry>
        <entry>read very_important_count (5)</entry>
       </row>
       <row>
        <entry>add 1 (6)</entry>
        <entry></entry>
       </row>
       <row>
        <entry></entry>
        <entry>add 1 (6)</entry>
       </row>
       <row>
        <entry>write very_important_count (6)</entry>
        <entry></entry>
       </row>
       <row>
        <entry></entry>
        <entry>write very_important_count (6)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <sect1 id="race-condition">
    <title>Race Conditions and Critical Regions</title>
    <para>
      This overlap, where the result depends on the
      relative timing of multiple tasks, is called a <firstterm>race condition</firstterm>.
      The piece of code containing the concurrency issue is called a
      <firstterm>critical region</firstterm>.  And especially since Linux starting running
      on SMP machines, they became one of the major issues in kernel
      design and implementation.
    </para>
    <para>
      Preemption can have the same effect, even if there is only one
      CPU: by preempting one task during the critical region, we have
      exactly the same race condition.  In this case the thread which
      preempts might run the critical region itself.
    </para>
    <para>
      The solution is to recognize when these simultaneous accesses
      occur, and use locks to make sure that only one instance can
      enter the critical region at any time.  There are many
      friendly primitives in the Linux kernel to help you do this.
      And then there are the unfriendly primitives, but I'll pretend
      they don't exist.
    </para>
    </sect1>
  </chapter>

  <chapter id="locks">
   <title>Locking in the Linux Kernel</title>

   <para>
     If I could give you one piece of advice: never sleep with anyone
     crazier than yourself.  But if I had to give you advice on
     locking: <emphasis>keep it simple</emphasis>.
   </para>

   <para>
     Be reluctant to introduce new locks.
   </para>

   <para>
     Strangely enough, this last one is the exact reverse of my advice when
     you <emphasis>have</emphasis> slept with someone crazier than yourself.
     And you should think about getting a big dog.
   </para>

   <sect1 id="lock-intro">
   <title>Two Main Types of Kernel Locks: Spinlocks and Mutexes</title>

   <para>
     There are two main types of kernel locks.  The fundamental type
     is the spinlock 
     (<filename class="headerfile">include/asm/spinlock.h</filename>),
     which is a very simple single-holder lock: if you can't get the 
     spinlock, you keep trying (spinning) until you can.  Spinlocks are 
     very small and fast, and can be used anywhere.
   </para>
   <para>
     The second type is a mutex
     (<filename class="headerfile">include/linux/mutex.h</filename>): it
     is like a spinlock, but you may block holding a mutex.
     If you can't lock a mutex, your task will suspend itself, and be woken
     up when the mutex is released.  This means the CPU can do something
     else while you are waiting.  There are many cases when you simply
     can't sleep (see <xref linkend="sleeping-things"/>), and so have to
     use a spinlock instead.
   </para>
   <para>
     Neither type of lock is recursive: see
     <xref linkend="deadlock"/>.
   </para>
   </sect1>
 
   <sect1 id="uniprocessor">
    <title>Locks and Uniprocessor Kernels</title>

    <para>
      For kernels compiled without <symbol>CONFIG_SMP</symbol>, and
      without <symbol>CONFIG_PREEMPT</symbol> spinlocks do not exist at
      all.  This is an excellent design decision: when no-one else can
      run at the same time, there is no reason to have a lock.
    </para>

    <para>
      If the kernel is compiled without <symbol>CONFIG_SMP</symbol>,
      but <symbol>CONFIG_PREEMPT</symbol> is set, then spinlocks
      simply disable preemption, which is sufficient to prevent any
      races.  For most purposes, we can think of preemption as
      equivalent to SMP, and not worry about it separately.
    </para>

    <para>
      You should always test your locking code with <symbol>CONFIG_SMP</symbol>
      and <symbol>CONFIG_PREEMPT</symbol> enabled, even if you don't have an SMP test box, because it
      will still catch some kinds of locking bugs.
    </para>

    <para>
      Mutexes still exist, because they are required for
      synchronization between <firstterm linkend="gloss-usercontext">user 
      contexts</firstterm>, as we will see below.
    </para>
   </sect1>

    <sect1 id="usercontextlocking">
     <title>Locking Only In User Context</title>

     <para>
       If you have a data structure which is only ever accessed from
       user context, then you can use a simple mutex
       (<filename>include/linux/mutex.h</filename>) to protect it.  This
       is the most trivial case: you initialize the mutex.  Then you can
       call <function>mutex_lock_interruptible()</function> to grab the mutex,
       and <function>mutex_unlock()</function> to release it.  There is also a 
       <function>mutex_lock()</function>, which should be avoided, because it 
       will not return if a signal is received.
     </para>

     <para>
       Example: <filename>net/netfilter/nf_sockopt.c</filename> allows 
       registration of new <function>setsockopt()</function> and 
       <function>getsockopt()</function> calls, with
       <function>nf_register_sockopt()</function>.  Registration and 
       de-registration are only done on module load and unload (and boot 
       time, where there is no concurrency), and the list of registrations 
       is only consulted for an unknown <function>setsockopt()</function>
       or <function>getsockopt()</function> system call.  The 
       <varname>nf_sockopt_mutex</varname> is perfect to protect this,
       especially since the setsockopt and getsockopt calls may well
       sleep.
     </para>
   </sect1>

   <sect1 id="lock-user-bh">
    <title>Locking Between User Context and Softirqs</title>

    <para>
      If a <firstterm linkend="gloss-softirq">softirq</firstterm> shares
      data with user context, you have two problems.  Firstly, the current 
      user context can be interrupted by a softirq, and secondly, the
      critical region could be entered from another CPU.  This is where
      <function>spin_lock_bh()</function> 
      (<filename class="headerfile">include/linux/spinlock.h</filename>) is
      used.  It disables softirqs on that CPU, then grabs the lock.
      <function>spin_unlock_bh()</function> does the reverse.  (The
      '_bh' suffix is a historical reference to "Bottom Halves", the
      old name for software interrupts.  It should really be
      called spin_lock_softirq()' in a perfect world).
    </para>

    <para>
      Note that you can also use <function>spin_lock_irq()</function>
      or <function>spin_lock_irqsave()</function> here, which stop
      hardware interrupts as well: see <xref linkend="hardirq-context"/>.
    </para>

    <para>
      This works perfectly for <firstterm linkend="gloss-up"><acronym>UP
      </acronym></firstterm> as well: the spin lock vanishes, and this macro 
      simply becomes <function>local_bh_disable()</function>
      (<filename class="headerfile">include/linux/interrupt.h</filename>), which
      protects you from the softirq being run.
    </para>
   </sect1>

   <sect1 id="lock-user-tasklet">
    <title>Locking Between User Context and Tasklets</title>

    <para>
      This is exactly the same as above, because <firstterm
      linkend="gloss-tasklet">tasklets</firstterm> are actually run
      from a softirq.
    </para>
   </sect1>

   <sect1 id="lock-user-timers">
    <title>Locking Between User Context and Timers</title>

    <para>
      This, too, is exactly the same as above, because <firstterm
      linkend="gloss-timers">timers</firstterm> are actually run from
      a softirq.  From a locking point of view, tasklets and timers
      are identical.
    </para>
   </sect1>

   <sect1 id="lock-tasklets">
    <title>Locking Between Tasklets/Timers</title>

    <para>
      Sometimes a tasklet or timer might want to share data with
      another tasklet or timer.
    </para>

    <sect2 id="lock-tasklets-same">
     <title>The Same Tasklet/Timer</title>
     <para>
       Since a tasklet is never run on two CPUs at once, you don't
       need to worry about your tasklet being reentrant (running
       twice at once), even on SMP.
     </para>
    </sect2>

    <sect2 id="lock-tasklets-different">
     <title>Different Tasklets/Timers</title>
     <para>
       If another tasklet/timer wants
       to share data with your tasklet or timer , you will both need to use
       <function>spin_lock()</function> and
       <function>spin_unlock()</function> calls.  
       <function>spin_lock_bh()</function> is
       unnecessary here, as you are already in a tasklet, and
       none will be run on the same CPU.
     </para>
    </sect2>
   </sect1>

   <sect1 id="lock-softirqs">
    <title>Locking Between Softirqs</title>

    <para>
      Often a softirq might
      want to share data with itself or a tasklet/timer.
    </para>

    <sect2 id="lock-softirqs-same">
     <title>The Same Softirq</title>

     <para>
       The same softirq can run on the other CPUs: you can use a
       per-CPU array (see <xref linkend="per-cpu"/>) for better
       performance.  If you're going so far as to use a softirq,
       you probably care about scalable performance enough
       to justify the extra complexity.
     </para>

     <para>
       You'll need to use <function>spin_lock()</function> and 
       <function>spin_unlock()</function> for shared data.
     </para>
    </sect2>

    <sect2 id="lock-softirqs-different">
     <title>Different Softirqs</title>

     <para>
       You'll need to use <function>spin_lock()</function> and
       <function>spin_unlock()</function> for shared data, whether it
       be a timer, tasklet, different softirq or the same or another
       softirq: any of them could be running on a different CPU.
     </para>
    </sect2>
   </sect1>
  </chapter>

  <chapter id="hardirq-context">
   <title>Hard IRQ Context</title>

   <para>
     Hardware interrupts usually communicate with a
     tasklet or softirq.  Frequently this involves putting work in a
     queue, which the softirq will take out.
   </para>

   <sect1 id="hardirq-softirq">
    <title>Locking Between Hard IRQ and Softirqs/Tasklets</title>

    <para>
      If a hardware irq handler shares data with a softirq, you have
      two concerns.  Firstly, the softirq processing can be
      interrupted by a hardware interrupt, and secondly, the
      critical region could be entered by a hardware interrupt on
      another CPU.  This is where <function>spin_lock_irq()</function> is 
      used.  It is defined to disable interrupts on that cpu, then grab 
      the lock. <function>spin_unlock_irq()</function> does the reverse.
    </para>

    <para>
      The irq handler does not to use
      <function>spin_lock_irq()</function>, because the softirq cannot
      run while the irq handler is running: it can use
      <function>spin_lock()</function>, which is slightly faster.  The
      only exception would be if a different hardware irq handler uses
      the same lock: <function>spin_lock_irq()</function> will stop
      that from interrupting us.
    </para>

    <para>
      This works perfectly for UP as well: the spin lock vanishes,
      and this macro simply becomes <function>local_irq_disable()</function>
      (<filename class="headerfile">include/asm/smp.h</filename>), which
      protects you from the softirq/tasklet/BH being run.
    </para>

    <para>
      <function>spin_lock_irqsave()</function> 
      (<filename>include/linux/spinlock.h</filename>) is a variant
      which saves whether interrupts were on or off in a flags word,
      which is passed to <function>spin_unlock_irqrestore()</function>.  This
      means that the same code can be used inside an hard irq handler (where
      interrupts are already off) and in softirqs (where the irq
      disabling is required).
    </para>

    <para>
      Note that softirqs (and hence tasklets and timers) are run on
      return from hardware interrupts, so
      <function>spin_lock_irq()</function> also stops these.  In that
      sense, <function>spin_lock_irqsave()</function> is the most
      general and powerful locking function.
    </para>

   </sect1>
   <sect1 id="hardirq-hardirq">
    <title>Locking Between Two Hard IRQ Handlers</title>
    <para>
      It is rare to have to share data between two IRQ handlers, but
      if you do, <function>spin_lock_irqsave()</function> should be
      used: it is architecture-specific whether all interrupts are
      disabled inside irq handlers themselves.
    </para>
   </sect1>

  </chapter>

  <chapter id="cheatsheet">
   <title>Cheat Sheet For Locking</title>
   <para>
     Pete Zaitcev gives the following summary:
   </para>
   <itemizedlist>
      <listitem>
	<para>
          If you are in a process context (any syscall) and want to
	lock other process out, use a mutex.  You can take a mutex
	and sleep (<function>copy_from_user*(</function> or
	<function>kmalloc(x,GFP_KERNEL)</function>).
      </para>
      </listitem>
      <listitem>
	<para>
	Otherwise (== data can be touched in an interrupt), use
	<function>spin_lock_irqsave()</function> and
	<function>spin_unlock_irqrestore()</function>.
	</para>
      </listitem>
      <listitem>
	<para>
	Avoid holding spinlock for more than 5 lines of code and
	across any function call (except accessors like
	<function>readb</function>).
	</para>
      </listitem>
    </itemizedlist>

   <sect1 id="minimum-lock-reqirements">
   <title>Table of Minimum Requirements</title>

   <para> The following table lists the <emphasis>minimum</emphasis>
	locking requirements between various contexts.  In some cases,
	the same context can only be running on one CPU at a time, so
	no locking is required for that context (eg. a particular
	thread can only run on one CPU at a time, but if it needs
	shares data with another thread, locking is required).
   </para>
   <para>
	Remember the advice above: you can always use
	<function>spin_lock_irqsave()</function>, which is a superset
	of all other spinlock primitives.
   </para>

   <table>
<title>Table of Locking Requirements</title>
<tgroup cols="11">
<tbody>

<row>
<entry></entry>
<entry>IRQ Handler A</entry>
<entry>IRQ Handler B</entry>
<entry>Softirq A</entry>
<entry>Softirq B</entry>
<entry>Tasklet A</entry>
<entry>Tasklet B</entry>
<entry>Timer A</entry>
<entry>Timer B</entry>
<entry>User Context A</entry>
<entry>User Context B</entry>
</row>

<row>
<entry>IRQ Handler A</entry>
<entry>None</entry>
</row>

<row>
<entry>IRQ Handler B</entry>
<entry>SLIS</entry>
<entry>None</entry>
</row>

<row>
<entry>Softirq A</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
</row>

<row>
<entry>Softirq B</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
<entry>SL</entry>
</row>

<row>
<entry>Tasklet A</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>None</entry>
</row>

<row>
<entry>Tasklet B</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>None</entry>
</row>

<row>
<entry>Timer A</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>None</entry>
</row>

<row>
<entry>Timer B</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>SL</entry>
<entry>None</entry>
</row>

<row>
<entry>User Context A</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>None</entry>
</row>

<row>
<entry>User Context B</entry>
<entry>SLI</entry>
<entry>SLI</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>SLBH</entry>
<entry>MLI</entry>
<entry>None</entry>
</row>

</tbody>
</tgroup>
</table>

   <table>
<title>Legend for Locking Requirements Table</title>
<tgroup cols="2">
<tbody>

<row>
<entry>SLIS</entry>
<entry>spin_lock_irqsave</entry>
</row>
<row>
<entry>SLI</entry>
<entry>spin_lock_irq</entry>
</row>
<row>
<entry>SL</entry>
<entry>spin_lock</entry>
</row>
<row>
<entry>SLBH</entry>
<entry>spin_lock_bh</entry>
</row>
<row>
<entry>MLI</entry>
<entry>mutex_lock_interruptible</entry>
</row>

</tbody>
</tgroup>
</table>

</sect1>
</chapter>

<chapter id="trylock-functions">
 <title>The trylock Functions</title>
  <para>
   There are functions that try to acquire a lock only once and immediately
   return a value telling about success or failure to acquire the lock.
   They can be used if you need no access to the data protected with the lock
   when some other thread is holding the lock. You should acquire the lock
   later if you then need access to the data protected with the lock.
  </para>

  <para>
    <function>spin_trylock()</function> does not spin but returns non-zero if
    it acquires the spinlock on the first try or 0 if not. This function can
    be used in all contexts like <function>spin_lock</function>: you must have
    disabled the contexts that might interrupt you and acquire the spin lock.
  </para>

  <para>
    <function>mutex_trylock()</function> does not suspend your task
    but returns non-zero if it could lock the mutex on the first try
    or 0 if not. This function cannot be safely used in hardware or software
    interrupt contexts despite not sleeping.
  </para>
</chapter>

  <chapter id="Examples">
   <title>Common Examples</title>
    <para>
Let's step through a simple example: a cache of number to name
mappings.  The cache keeps a count of how often each of the objects is
used, and when it gets full, throws out the least used one.

    </para>

   <sect1 id="examples-usercontext">
    <title>All In User Context</title>
    <para>
For our first example, we assume that all operations are in user
context (ie. from system calls), so we can sleep.  This means we can
use a mutex to protect the cache and all the objects within
it.  Here's the code:
    </para>

    <programlisting>
#include &lt;linux/list.h&gt;
#include &lt;linux/slab.h&gt;
#include &lt;linux/string.h&gt;
#include &lt;linux/mutex.h&gt;
#include &lt;asm/errno.h&gt;

struct object
{
        struct list_head list;
        int id;
        char name[32];
        int popularity;
};

/* Protects the cache, cache_num, and the objects within it */
static DEFINE_MUTEX(cache_lock);
static LIST_HEAD(cache);
static unsigned int cache_num = 0;
#define MAX_CACHE_SIZE 10

/* Must be holding cache_lock */
static struct object *__cache_find(int id)
{
        struct object *i;

        list_for_each_entry(i, &amp;cache, list)
                if (i-&gt;id == id) {
                        i-&gt;popularity++;
                        return i;
                }
        return NULL;
}

/* Must be holding cache_lock */
static void __cache_delete(struct object *obj)
{
        BUG_ON(!obj);
        list_del(&amp;obj-&gt;list);
        kfree(obj);
        cache_num--;
}

/* Must be holding cache_lock */
static void __cache_add(struct object *obj)
{
        list_add(&amp;obj-&gt;list, &amp;cache);
        if (++cache_num > MAX_CACHE_SIZE) {
                struct object *i, *outcast = NULL;
                list_for_each_entry(i, &amp;cache, list) {
                        if (!outcast || i-&gt;popularity &lt; outcast-&gt;popularity)
                                outcast = i;
                }
                __cache_delete(outcast);
        }
}

int cache_add(int id, const char *name)
{
        struct object *obj;

        if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
                return -ENOMEM;

        strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
        obj-&gt;id = id;
        obj-&gt;popularity = 0;

        mutex_lock(&amp;cache_lock);
        __cache_add(obj);
        mutex_unlock(&amp;cache_lock);
        return 0;
}

void cache_delete(int id)
{
        mutex_lock(&amp;cache_lock);
        __cache_delete(__cache_find(id));
        mutex_unlock(&amp;cache_lock);
}

int cache_find(int id, char *name)
{
        struct object *obj;
        int ret = -ENOENT;

        mutex_lock(&amp;cache_lock);
        obj = __cache_find(id);
        if (obj) {
                ret = 0;
                strcpy(name, obj-&gt;name);
        }
        mutex_unlock(&amp;cache_lock);
        return ret;
}
</programlisting>

    <para>
Note that we always make sure we have the cache_lock when we add,
delete, or look up the cache: both the cache infrastructure itself and
the contents of the objects are protected by the lock.  In this case
it's easy, since we copy the data for the user, and never let them
access the objects directly.
    </para>
    <para>
There is a slight (and common) optimization here: in
<function>cache_add</function> we set up the fields of the object
before grabbing the lock.  This is safe, as no-one else can access it
until we put it in cache.
    </para>
    </sect1>

   <sect1 id="examples-interrupt">
    <title>Accessing From Interrupt Context</title>
    <para>
Now consider the case where <function>cache_find</function> can be
called from interrupt context: either a hardware interrupt or a
softirq.  An example would be a timer which deletes object from the
cache.
    </para>
    <para>
The change is shown below, in standard patch format: the
<symbol>-</symbol> are lines which are taken away, and the
<symbol>+</symbol> are lines which are added.
    </para>
<programlisting>
--- cache.c.usercontext	2003-12-09 13:58:54.000000000 +1100
+++ cache.c.interrupt	2003-12-09 14:07:49.000000000 +1100
@@ -12,7 +12,7 @@
         int popularity;
 };

-static DEFINE_MUTEX(cache_lock);
+static DEFINE_SPINLOCK(cache_lock);
 static LIST_HEAD(cache);
 static unsigned int cache_num = 0;
 #define MAX_CACHE_SIZE 10
@@ -55,6 +55,7 @@
 int cache_add(int id, const char *name)
 {
         struct object *obj;
+        unsigned long flags;

         if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL)
                 return -ENOMEM;
@@ -63,30 +64,33 @@
         obj-&gt;id = id;
         obj-&gt;popularity = 0;

-        mutex_lock(&amp;cache_lock);
+        spin_lock_irqsave(&amp;cache_lock, flags);
         __cache_add(obj);
-        mutex_unlock(&amp;cache_lock);
+        spin_unlock_irqrestore(&amp;cache_lock, flags);
         return 0;
 }

 void cache_delete(int id)
 {
-        mutex_lock(&amp;cache_lock);
+        unsigned long flags;
+
+        spin_lock_irqsave(&amp;cache_lock, flags);
         __cache_delete(__cache_find(id));
-        mutex_unlock(&amp;cache_lock);
+        spin_unlock_irqrestore(&amp;cache_lock, flags);
 }

 int cache_find(int id, char *name)
 {
         struct object *obj;
         int ret = -ENOENT;
+        unsigned long flags;

-        mutex_lock(&amp;cache_lock);
+        spin_lock_irqsave(&amp;cache_lock, flags);
         obj = __cache_find(id);
         if (obj) {
                 ret = 0;
                 strcpy(name, obj-&gt;name);
         }
-        mutex_unlock(&amp;cache_lock);
+        spin_unlock_irqrestore(&amp;cache_lock, flags);
         return ret;
 }
</programlisting>

    <para>
Note that the <function>spin_lock_irqsave</function> will turn off
interrupts if they are on, otherwise does nothing (if we are already
in an interrupt handler), hence these functions are safe to call from
any context.
    </para>
    <para>
Unfortunately, <function>cache_add</function> calls
<function>kmalloc</function> with the <symbol>GFP_KERNEL</symbol>
flag, which is only legal in user context.  I have assumed that
<function>cache_add</function> is still only called in user context,
otherwise this should become a parameter to
<function>cache_add</function>.
    </para>
  </sect1>
   <sect1 id="examples-refcnt">
    <title>Exposing Objects Outside This File</title>
    <para>
If our objects contained more information, it might not be sufficient
to copy the information in and out: other parts of the code might want
to keep pointers to these objects, for example, rather than looking up
the id every time.  This produces two problems.
    </para>
    <para>
The first problem is that we use the <symbol>cache_lock</symbol> to
protect objects: we'd need to make this non-static so the rest of the
code can use it.  This makes locking trickier, as it is no longer all
in one place.
    </para>
    <para>
The second problem is the lifetime problem: if another structure keeps
a pointer to an object, it presumably expects that pointer to remain
valid.  Unfortunately, this is only guaranteed while you hold the
lock, otherwise someone might call <function>cache_delete</function>
and even worse, add another object, re-using the same address.
    </para>
    <para>
As there is only one lock, you can't hold it forever: no-one else would
get any work done.
    </para>
    <para>
The solution to this problem is to use a reference count: everyone who
has a pointer to the object increases it when they first get the
object, and drops the reference count when they're finished with it.
Whoever drops it to zero knows it is unused, and can actually delete it.
    </para>
    <para>
Here is the code:
    </para>

<programlisting>
--- cache.c.interrupt	2003-12-09 14:25:43.000000000 +1100
+++ cache.c.refcnt	2003-12-09 14:33:05.000000000 +1100
@@ -7,6 +7,7 @@
 struct object
 {
         struct list_head list;
+        unsigned int refcnt;
         int id;
         char name[32];
         int popularity;
@@ -17,6 +18,35 @@
 static unsigned int cache_num = 0;
 #define MAX_CACHE_SIZE 10

+static void __object_put(struct object *obj)
+{
+        if (--obj-&gt;refcnt == 0)
+                kfree(obj);
+}
+
+static void __object_get(struct object *obj)
+{
+        obj-&gt;refcnt++;
+}
+
+void object_put(struct object *obj)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&amp;cache_lock, flags);
+        __object_put(obj);
+        spin_unlock_irqrestore(&amp;cache_lock, flags);
+}
+
+void object_get(struct object *obj)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&amp;cache_lock, flags);
+        __object_get(obj);
+        spin_unlock_irqrestore(&amp;cache_lock, flags);
+}
+
 /* Must be holding cache_lock */
 static struct object *__cache_find(int id)
 {
@@ -35,6 +65,7 @@
 {
         BUG_ON(!obj);
         list_del(&amp;obj-&gt;list);
+        __object_put(obj);
         cache_num--;
 }

@@ -63,6 +94,7 @@
         strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
         obj-&gt;id = id;
         obj-&gt;popularity = 0;
+        obj-&gt;refcnt = 1; /* The cache holds a reference */

         spin_lock_irqsave(&amp;cache_lock, flags);
         __cache_add(obj);
@@ -79,18 +111,15 @@
         spin_unlock_irqrestore(&amp;cache_lock, flags);
 }

-int cache_find(int id, char *name)
+struct object *cache_find(int id)
 {
         struct object *obj;
-        int ret = -ENOENT;
         unsigned long flags;

         spin_lock_irqsave(&amp;cache_lock, flags);
         obj = __cache_find(id);
-        if (obj) {
-                ret = 0;
-                strcpy(name, obj-&gt;name);
-        }
+        if (obj)
+                __object_get(obj);
         spin_unlock_irqrestore(&amp;cache_lock, flags);
-        return ret;
+        return obj;
 }
</programlisting>

<para>
We encapsulate the reference counting in the standard 'get' and 'put'
functions.  Now we can return the object itself from
<function>cache_find</function> which has the advantage that the user
can now sleep holding the object (eg. to
<function>copy_to_user</function> to name to userspace).
</para>
<para>
The other point to note is that I said a reference should be held for
every pointer to the object: thus the reference count is 1 when first
inserted into the cache.  In some versions the framework does not hold
a reference count, but they are more complicated.
</para>

   <sect2 id="examples-refcnt-atomic">
    <title>Using Atomic Operations For The Reference Count</title>
<para>
In practice, <type>atomic_t</type> would usually be used for
<structfield>refcnt</structfield>.  There are a number of atomic
operations defined in

<filename class="headerfile">include/asm/atomic.h</filename>: these are
guaranteed to be seen atomically from all CPUs in the system, so no
lock is required.  In this case, it is simpler than using spinlocks,
although for anything non-trivial using spinlocks is clearer.  The
<function>atomic_inc</function> and
<function>atomic_dec_and_test</function> are used instead of the
standard increment and decrement operators, and the lock is no longer
used to protect the reference count itself.
</para>

<programlisting>
--- cache.c.refcnt	2003-12-09 15:00:35.000000000 +1100
+++ cache.c.refcnt-atomic	2003-12-11 15:49:42.000000000 +1100
@@ -7,7 +7,7 @@
 struct object
 {
         struct list_head list;
-        unsigned int refcnt;
+        atomic_t refcnt;
         int id;
         char name[32];
         int popularity;
@@ -18,33 +18,15 @@
 static unsigned int cache_num = 0;
 #define MAX_CACHE_SIZE 10

-static void __object_put(struct object *obj)
-{
-        if (--obj-&gt;refcnt == 0)
-                kfree(obj);
-}
-
-static void __object_get(struct object *obj)
-{
-        obj-&gt;refcnt++;
-}
-
 void object_put(struct object *obj)
 {
-        unsigned long flags;
-
-        spin_lock_irqsave(&amp;cache_lock, flags);
-        __object_put(obj);
-        spin_unlock_irqrestore(&amp;cache_lock, flags);
+        if (atomic_dec_and_test(&amp;obj-&gt;refcnt))
+                kfree(obj);
 }

 void object_get(struct object *obj)
 {
-        unsigned long flags;
-
-        spin_lock_irqsave(&amp;cache_lock, flags);
-        __object_get(obj);
-        spin_unlock_irqrestore(&amp;cache_lock, flags);
+        atomic_inc(&amp;obj-&gt;refcnt);
 }

 /* Must be holding cache_lock */
@@ -65,7 +47,7 @@
 {
         BUG_ON(!obj);
         list_del(&amp;obj-&gt;list);
-        __object_put(obj);
+        object_put(obj);
         cache_num--;
 }

@@ -94,7 +76,7 @@
         strlcpy(obj-&gt;name, name, sizeof(obj-&gt;name));
         obj-&gt;id = id;
         obj-&gt;popularity = 0;
-        obj-&gt;refcnt = 1; /* The cache holds a reference */
+        atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */

         spin_lock_irqsave(&amp;cache_lock, flags);
         __cache_add(obj);
@@ -119,7 +101,7 @@
         spin_lock_irqsave(&amp;cache_lock, flags);
         obj = __cache_find(id);
         if (obj)
-                __object_get(obj);
+                object_get(obj);
         spin_unlock_irqrestore(&amp;cache_lock, flags);
         return obj;
 }
</programlisting>
</sect2>
</sect1>

   <sect1 id="examples-lock-per-obj">
    <title>Protecting The Objects Themselves</title>
    <para>
In these examples, we assumed that the objects (except the reference
counts) never changed once they are created.  If we wanted to allow
the name to change, there are three possibilities:
    </para>
    <itemizedlist>
      <listitem>
	<para>
You can make <symbol>cache_lock</symbol> non-static, and tell people
to grab that lock before changing the name in any object.
        </para>
      </listitem>
      <listitem>
        <para>
You can provide a <function>cache_obj_rename</function> which grabs
this lock and changes the name for the caller, and tell everyone to
use that function.
        </para>
      </listitem>
      <listitem>
        <para>
You can make the <symbol>cache_lock</symbol> protect only the cache
itself, and use another lock to protect the name.
        </para>
      </listitem>
    </itemizedlist>

      <para>
Theoretically, you can make the locks as fine-grained as one lock for
every field, for every object.  In practice, the most common variants
are:
</para>
    <itemizedlist>
      <listitem>
	<para>
One lock which protects the infrastructure (the <symbol>cache</symbol>
list in this example) and all the objects.  This is what we have done
so far.
	</para>
      </listitem>
      <listitem>
        <para>
One lock which protects the infrastructure (including the list
pointers inside the objects), and one lock inside the object which
protects the rest of that object.
        </para>
      </listitem>
      <listitem>
        <para>
Multiple locks to protect the infrastructure (eg. one lock per hash
chain), possibly with a separate per-object lock.
        </para>
      </listitem>
    </itemizedlist>

<para>
Here is the "lock-per-object" implementation:
</para>
<programlisting>
--- cache.c.refcnt-atomic	2003-12-11 15:50:54.000000000 +1100
+++ cache.c.perobjectlock	2003-12-11 17:15:03.000000000 +1100
@@ -6,11 +6,17 @@

 struct object
 {
+        /* These two protected by cache_lock. */
         struct list_head list;
+        int popularity;
+
         atomic_t refcnt;
+
+        /* Doesn't change once created. */
         int id;
+
+        spinlock_t lock; /* Protects the name */
         char name[32];
-        int popularity;
 };

 static DEFINE_SPINLOCK(cache_lock);
@@ -77,6 +84,7 @@
         obj-&gt;id = id;
         obj-&gt;popularity = 0;
         atomic_set(&amp;obj-&gt;refcnt, 1); /* The cache holds a reference */
+        spin_lock_init(&amp;obj-&gt;lock);

         spin_lock_irqsave(&amp;cache_lock, flags);
         __cache_add(obj);
</programlisting>

<para>
Note that I decide that the <structfield>popularity</structfield>
count should be protected by the <symbol>cache_lock</symbol> rather
than the per-object lock: this is because it (like the
<structname>struct list_head</structname> inside the object) is
logically part of the infrastructure.  This way, I don't need to grab
the lock of every object in <function>__cache_add</function> when
seeking the least popular.
</para>

<para>
I also decided that the <structfield>id</structfield> member is
unchangeable, so I don't need to grab each object lock in
<function>__cache_find()</function> to examine the
<structfield>id</structfield>: the object lock is only used by a
caller who wants to read or write the <structfield>name</structfield>
field.
</para>

<para>
Note also that I added a comment describing what data was protected by
which locks.  This is extremely important, as it describes the runtime
behavior of the code, and can be hard to gain from just reading.  And
as Alan Cox says, <quote>Lock data, not code</quote>.
</para>
</sect1>
</chapter>

   <chapter id="common-problems">
    <title>Common Problems</title>
    <sect1 id="deadlock">
    <title>Deadlock: Simple and Advanced</title>

    <para>
      There is a coding bug where a piece of code tries to grab a
      spinlock twice: it will spin forever, waiting for the lock to
      be released (spinlocks, rwlocks and mutexes are not
      recursive in Linux).  This is trivial to diagnose: not a
      stay-up-five-nights-talk-to-fluffy-code-bunnies kind of
      problem.
    </para>

    <para>
      For a slightly more complex case, imagine you have a region
      shared by a softirq and user context.  If you use a
      <function>spin_lock()</function> call to protect it, it is 
      possible that the user context will be interrupted by the softirq
      while it holds the lock, and the softirq will then spin
      forever trying to get the same lock.
    </para>

    <para>
      Both of these are called deadlock, and as shown above, it can
      occur even with a single CPU (although not on UP compiles,
      since spinlocks vanish on kernel compiles with 
      <symbol>CONFIG_SMP</symbol>=n. You'll still get data corruption 
      in the second example).
    </para>

    <para>
      This complete lockup is easy to diagnose: on SMP boxes the
      watchdog timer or compiling with <symbol>DEBUG_SPINLOCK</symbol> set
      (<filename>include/linux/spinlock.h</filename>) will show this up 
      immediately when it happens.
    </para>

    <para>
      A more complex problem is the so-called 'deadly embrace',
      involving two or more locks.  Say you have a hash table: each
      entry in the table is a spinlock, and a chain of hashed
      objects.  Inside a softirq handler, you sometimes want to
      alter an object from one place in the hash to another: you
      grab the spinlock of the old hash chain and the spinlock of
      the new hash chain, and delete the object from the old one,
      and insert it in the new one.
    </para>

    <para>
      There are two problems here.  First, if your code ever
      tries to move the object to the same chain, it will deadlock
      with itself as it tries to lock it twice.  Secondly, if the
      same softirq on another CPU is trying to move another object
      in the reverse direction, the following could happen:
    </para>

    <table>
     <title>Consequences</title>

     <tgroup cols="2" align="left">

      <thead>
       <row>
        <entry>CPU 1</entry>
        <entry>CPU 2</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>Grab lock A -&gt; OK</entry>
        <entry>Grab lock B -&gt; OK</entry>
       </row>
       <row>
        <entry>Grab lock B -&gt; spin</entry>
        <entry>Grab lock A -&gt; spin</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
      The two CPUs will spin forever, waiting for the other to give up
      their lock.  It will look, smell, and feel like a crash.
    </para>
    </sect1>

    <sect1 id="techs-deadlock-prevent">
     <title>Preventing Deadlock</title>

     <para>
       Textbooks will tell you that if you always lock in the same
       order, you will never get this kind of deadlock.  Practice
       will tell you that this approach doesn't scale: when I
       create a new lock, I don't understand enough of the kernel
       to figure out where in the 5000 lock hierarchy it will fit.
     </para>

     <para>
       The best locks are encapsulated: they never get exposed in
       headers, and are never held around calls to non-trivial
       functions outside the same file.  You can read through this
       code and see that it will never deadlock, because it never
       tries to grab another lock while it has that one.  People
       using your code don't even need to know you are using a
       lock.
     </para>

     <para>
       A classic problem here is when you provide callbacks or
       hooks: if you call these with the lock held, you risk simple
       deadlock, or a deadly embrace (who knows what the callback
       will do?).  Remember, the other programmers are out to get
       you, so don't do this.
     </para>

    <sect2 id="techs-deadlock-overprevent">
     <title>Overzealous Prevention Of Deadlocks</title>

     <para>
       Deadlocks are problematic, but not as bad as data
       corruption.  Code which grabs a read lock, searches a list,
       fails to find what it wants, drops the read lock, grabs a
       write lock and inserts the object has a race condition.
     </para>

     <para>
       If you don't see why, please stay the fuck away from my code.
     </para>
    </sect2>
    </sect1>

   <sect1 id="racing-timers">
    <title>Racing Timers: A Kernel Pastime</title>

    <para>
      Timers can produce their own special problems with races.
      Consider a collection of objects (list, hash, etc) where each
      object has a timer which is due to destroy it.
    </para>

    <para>
      If you want to destroy the entire collection (say on module
      removal), you might do the following:
    </para>

    <programlisting>
        /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE
           HUNGARIAN NOTATION */
        spin_lock_bh(&amp;list_lock);

        while (list) {
                struct foo *next = list-&gt;next;
                del_timer(&amp;list-&gt;timer);
                kfree(list);
                list = next;
        }

        spin_unlock_bh(&amp;list_lock);
    </programlisting>

    <para>
      Sooner or later, this will crash on SMP, because a timer can
      have just gone off before the <function>spin_lock_bh()</function>,
      and it will only get the lock after we
      <function>spin_unlock_bh()</function>, and then try to free
      the element (which has already been freed!).
    </para>

    <para>
      This can be avoided by checking the result of
      <function>del_timer()</function>: if it returns
      <returnvalue>1</returnvalue>, the timer has been deleted.
      If <returnvalue>0</returnvalue>, it means (in this
      case) that it is currently running, so we can do:
    </para>

    <programlisting>
        retry:
                spin_lock_bh(&amp;list_lock);

                while (list) {
                        struct foo *next = list-&gt;next;
                        if (!del_timer(&amp;list-&gt;timer)) {
                                /* Give timer a chance to delete this */
                                spin_unlock_bh(&amp;list_lock);
                                goto retry;
                        }
                        kfree(list);
                        list = next;
                }

                spin_unlock_bh(&amp;list_lock);
    </programlisting>

    <para>
      Another common problem is deleting timers which restart
      themselves (by calling <function>add_timer()</function> at the end
      of their timer function).  Because this is a fairly common case
      which is prone to races, you should use <function>del_timer_sync()</function>
      (<filename class="headerfile">include/linux/timer.h</filename>)
      to handle this case.  It returns the number of times the timer
      had to be deleted before we finally stopped it from adding itself back
      in.
    </para>
   </sect1>

  </chapter>

 <chapter id="Efficiency">
    <title>Locking Speed</title>

    <para>
There are three main things to worry about when considering speed of
some code which does locking.  First is concurrency: how many things
are going to be waiting while someone else is holding a lock.  Second
is the time taken to actually acquire and release an uncontended lock.
Third is using fewer, or smarter locks.  I'm assuming that the lock is
used fairly often: otherwise, you wouldn't be concerned about
efficiency.
</para>
    <para>
Concurrency depends on how long the lock is usually held: you should
hold the lock for as long as needed, but no longer.  In the cache
example, we always create the object without the lock held, and then
grab the lock only when we are ready to insert it in the list.
</para>
    <para>
Acquisition times depend on how much damage the lock operations do to
the pipeline (pipeline stalls) and how likely it is that this CPU was
the last one to grab the lock (ie. is the lock cache-hot for this
CPU): on a machine with more CPUs, this likelihood drops fast.
Consider a 700MHz Intel Pentium III: an instruction takes about 0.7ns,
an atomic increment takes about 58ns, a lock which is cache-hot on
this CPU takes 160ns, and a cacheline transfer from another CPU takes
an additional 170 to 360ns.  (These figures from Paul McKenney's
<ulink url="http://www.linuxjournal.com/article.php?sid=6993"> Linux
Journal RCU article</ulink>).
</para>
    <para>
These two aims conflict: holding a lock for a short time might be done
by splitting locks into parts (such as in our final per-object-lock
example), but this increases the number of lock acquisitions, and the
results are often slower than having a single lock.  This is another
reason to advocate locking simplicity.
</para>
    <para>
The third concern is addressed below: there are some methods to reduce
the amount of locking which needs to be done.
</para>

  <sect1 id="efficiency-rwlocks">
   <title>Read/Write Lock Variants</title>

   <para>
      Both spinlocks and mutexes have read/write variants:
      <type>rwlock_t</type> and <structname>struct rw_semaphore</structname>.
      These divide users into two classes: the readers and the writers.  If
      you are only reading the data, you can get a read lock, but to write to
      the data you need the write lock.  Many people can hold a read lock,
      but a writer must be sole holder.
    </para>

   <para>
      If your code divides neatly along reader/writer lines (as our
      cache code does), and the lock is held by readers for
      significant lengths of time, using these locks can help.  They
      are slightly slower than the normal locks though, so in practice
      <type>rwlock_t</type> is not usually worthwhile.
    </para>
   </sect1>

   <sect1 id="efficiency-read-copy-update">
    <title>Avoiding Locks: Read Copy Update</title>

    <para>
      There is a special method of read/write locking called Read Copy
      Update.  Using RCU, the readers can avoid taking a lock
      altogether: as we expect our cache to be read more often than
      updated (otherwise the cache is a waste of time), it is a
      candidate for this optimization.
    </para>

    <para>
      How do we get rid of read locks?  Getting rid of read locks
      means that writers may be changing the list underneath the
      readers.  That is actually quite simple: we can read a linked
      list while an element is being added if the writer adds the
      element very carefully.  For example, adding
      <symbol>new</symbol> to a single linked list called
      <symbol>list</symbol>:
    </para>

    <programlisting>
        new-&gt;next = list-&gt;next;
        wmb();
        list-&gt;next = new;
    </programlisting>

    <para>
      The <function>wmb()</function> is a write memory barrier.  It
      ensures that the first operation (setting the new element's
      <symbol>next</symbol> pointer) is complete and will be seen by
      all CPUs, before the second operation is (putting the new
      element into the list).  This is important, since modern
      compilers and modern CPUs can both reorder instructions unless
      told otherwise: we want a reader to either not see the new
      element at all, or see the new element with the
      <symbol>next</symbol> pointer correctly pointing at the rest of
      the list.
    </para>
    <para>
      Fortunately, there is a function to do this for standard
      <structname>struct list_head</structname> lists:
      <function>list_add_rcu()</function>
      (<filename>include/linux/list.h</filename>).
    </para>
    <para>
      Removing an element from the list is even simpler: we replace
      the pointer to the old element with a pointer to its successor,
      and readers will either see it, or skip over it.
    </para>
    <programlisting>
        list-&gt;next = old-&gt;next;
    </programlisting>
    <para>
      There is <function>list_del_rcu()</function>
      (<filename>include/linux/list.h</filename>) which does this (the
      normal version poisons the old object, which we don't want).
    </para>
    <para>
      The reader must also be careful: some CPUs can look through the
      <symbol>next</symbol> pointer to start reading the contents of
      the next element early, but don't realize that the pre-fetched
      contents is wrong when the <symbol>next</symbol> pointer changes
      underneath them.  Once again, there is a
      <function>list_for_each_entry_rcu()</function>
      (<filename>include/linux/list.h</filename>) to help you.  Of
      course, writers can just use
      <function>list_for_each_entry()</function>, since there cannot
      be two simultaneous writers.
    </para>
    <para>
      Our final dilemma is this: when can we actually destroy the
      removed element?  Remember, a reader might be stepping through
      this element in the list right now: if we free this element and
      the <symbol>next</symbol> pointer changes, the reader will jump
      off into garbage and crash.  We need to wait until we know that
      all the readers who were traversing the list when we deleted the
      element are finished.  We use <function>call_rcu()</function> to
      register a callback which will actually destroy the object once
      all pre-existing readers are finished.  Alternatively,
      <function>synchronize_rcu()</function> may be used to block until
      all pre-existing are finished.
    </para>
    <para>
      But how does Read Copy Update know when the readers are
      finished?  The method is this: firstly, the readers always
      traverse the list inside
      <function>rcu_read_lock()</function>/<function>rcu_read_unlock()</function>
      pairs: these simply disable preemption so the reader won't go to
      sleep while reading the list.
    </para>
    <para>
      RCU then waits until every other CPU has slept at least once:
      since readers cannot sleep, we know that any readers which were
      traversing the list during the deletion are finished, and the
      callback is triggered.  The real Read Copy Update code is a
      little more optimized than this, but this is the fundamental
      idea.
    </para>

<programlisting>
--- cache.c.perobjectlock	2003-12-11 17:15:03.000000000 +1100
+++ cache.c.rcupdate	2003-12-11 17:55:14.000000000 +1100
@@ -1,15 +1,18 @@
 #include &lt;linux/list.h&gt;
 #include &lt;linux/slab.h&gt;
 #include &lt;linux/string.h&gt;
+#include &lt;linux/rcupdate.h&gt;
 #include &lt;linux/mutex.h&gt;
 #include &lt;asm/errno.h&gt;

 struct object
 {
-        /* These two protected by cache_lock. */
+        /* This is protected by RCU */
         struct list_head list;
         int popularity;

+        struct rcu_head rcu;
+
         atomic_t refcnt;

         /* Doesn't change once created. */
@@ -40,7 +43,7 @@
 {
         struct object *i;

-        list_for_each_entry(i, &amp;cache, list) {
+        list_for_each_entry_rcu(i, &amp;cache, list) {
                 if (i-&gt;id == id) {
                         i-&gt;popularity++;
                         return i;
@@ -49,19 +52,25 @@
         return NULL;
 }

+/* Final discard done once we know no readers are looking. */
+static void cache_delete_rcu(void *arg)
+{
+        object_put(arg);
+}
+
 /* Must be holding cache_lock */
 static void __cache_delete(struct object *obj)
 {
         BUG_ON(!obj);
-        list_del(&amp;obj-&gt;list);
-        object_put(obj);
+        list_del_rcu(&amp;obj-&gt;list);
         cache_num--;
+        call_rcu(&amp;obj-&gt;rcu, cache_delete_rcu);
 }

 /* Must be holding cache_lock */
 static void __cache_add(struct object *obj)
 {
-        list_add(&amp;obj-&gt;list, &amp;cache);
+        list_add_rcu(&amp;obj-&gt;list, &amp;cache);
         if (++cache_num > MAX_CACHE_SIZE) {
                 struct object *i, *outcast = NULL;
                 list_for_each_entry(i, &amp;cache, list) {
@@ -104,12 +114,11 @@
 struct object *cache_find(int id)
 {
         struct object *obj;
-        unsigned long flags;

-        spin_lock_irqsave(&amp;cache_lock, flags);
+        rcu_read_lock();
         obj = __cache_find(id);
         if (obj)
                 object_get(obj);
-        spin_unlock_irqrestore(&amp;cache_lock, flags);
+        rcu_read_unlock();
         return obj;
 }
</programlisting>

<para>
Note that the reader will alter the
<structfield>popularity</structfield> member in
<function>__cache_find()</function>, and now it doesn't hold a lock.
One solution would be to make it an <type>atomic_t</type>, but for
this usage, we don't really care about races: an approximate result is
good enough, so I didn't change it.
</para>

<para>
The result is that <function>cache_find()</function> requires no
synchronization with any other functions, so is almost as fast on SMP
as it would be on UP.
</para>

<para>
There is a furthur optimization possible here: remember our original
cache code, where there were no reference counts and the caller simply
held the lock whenever using the object?  This is still possible: if
you hold the lock, no one can delete the object, so you don't need to
get and put the reference count.
</para>

<para>
Now, because the 'read lock' in RCU is simply disabling preemption, a
caller which always has preemption disabled between calling
<function>cache_find()</function> and
<function>object_put()</function> does not need to actually get and
put the reference count: we could expose
<function>__cache_find()</function> by making it non-static, and
such callers could simply call that.
</para>
<para>
The benefit here is that the reference count is not written to: the
object is not altered in any way, which is much faster on SMP
machines due to caching.
</para>
  </sect1>

   <sect1 id="per-cpu">
    <title>Per-CPU Data</title>

    <para>
      Another technique for avoiding locking which is used fairly
      widely is to duplicate information for each CPU.  For example,
      if you wanted to keep a count of a common condition, you could
      use a spin lock and a single counter.  Nice and simple.
    </para>

    <para>
      If that was too slow (it's usually not, but if you've got a
      really big machine to test on and can show that it is), you
      could instead use a counter for each CPU, then none of them need
      an exclusive lock.  See <function>DEFINE_PER_CPU()</function>,
      <function>get_cpu_var()</function> and
      <function>put_cpu_var()</function>
      (<filename class="headerfile">include/linux/percpu.h</filename>).
    </para>

    <para>
      Of particular use for simple per-cpu counters is the
      <type>local_t</type> type, and the
      <function>cpu_local_inc()</function> and related functions,
      which are more efficient than simple code on some architectures
      (<filename class="headerfile">include/asm/local.h</filename>).
    </para>

    <para>
      Note that there is no simple, reliable way of getting an exact
      value of such a counter, without introducing more locks.  This
      is not a problem for some uses.
    </para>
   </sect1>

   <sect1 id="mostly-hardirq">
    <title>Data Which Mostly Used By An IRQ Handler</title>

    <para>
      If data is always accessed from within the same IRQ handler, you
      don't need a lock at all: the kernel already guarantees that the
      irq handler will not run simultaneously on multiple CPUs.
    </para>
    <para>
      Manfred Spraul points out that you can still do this, even if
      the data is very occasionally accessed in user context or
      softirqs/tasklets.  The irq handler doesn't use a lock, and
      all other accesses are done as so:
    </para>

<programlisting>
	spin_lock(&amp;lock);
	disable_irq(irq);
	...
	enable_irq(irq);
	spin_unlock(&amp;lock);
</programlisting>
    <para>
      The <function>disable_irq()</function> prevents the irq handler
      from running (and waits for it to finish if it's currently
      running on other CPUs).  The spinlock prevents any other
      accesses happening at the same time.  Naturally, this is slower
      than just a <function>spin_lock_irq()</function> call, so it
      only makes sense if this type of access happens extremely
      rarely.
    </para>
   </sect1>
  </chapter>

 <chapter id="sleeping-things">
    <title>What Functions Are Safe To Call From Interrupts?</title>

    <para>
      Many functions in the kernel sleep (ie. call schedule())
      directly or indirectly: you can never call them while holding a
      spinlock, or with preemption disabled.  This also means you need
      to be in user context: calling them from an interrupt is illegal.
    </para>

   <sect1 id="sleeping">
    <title>Some Functions Which Sleep</title>

    <para>
      The most common ones are listed below, but you usually have to
      read the code to find out if other calls are safe.  If everyone
      else who calls it can sleep, you probably need to be able to
      sleep, too.  In particular, registration and deregistration
      functions usually expect to be called from user context, and can
      sleep.
    </para>

    <itemizedlist>
     <listitem>
      <para>
        Accesses to 
        <firstterm linkend="gloss-userspace">userspace</firstterm>:
      </para>
      <itemizedlist>
       <listitem>
        <para>
          <function>copy_from_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function>copy_to_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function>get_user()</function>
        </para>
       </listitem>
       <listitem>
        <para>
          <function>put_user()</function>
        </para>
       </listitem>
      </itemizedlist>
     </listitem>

     <listitem>
      <para>
        <function>kmalloc(GFP_KERNEL)</function>
      </para>
     </listitem>

     <listitem>
      <para>
      <function>mutex_lock_interruptible()</function> and
      <function>mutex_lock()</function>
      </para>
      <para>
       There is a <function>mutex_trylock()</function> which does not
       sleep.  Still, it must not be used inside interrupt context since
       its implementation is not safe for that.
       <function>mutex_unlock()</function> will also never sleep.
       It cannot be used in interrupt context either since a mutex
       must be released by the same task that acquired it.
      </para>
     </listitem>
    </itemizedlist>
   </sect1>

   <sect1 id="dont-sleep">
    <title>Some Functions Which Don't Sleep</title>

    <para>
     Some functions are safe to call from any context, or holding
     almost any lock.
    </para>

    <itemizedlist>
     <listitem>
      <para>
	<function>printk()</function>
      </para>
     </listitem>
     <listitem>
      <para>
        <function>kfree()</function>
      </para>
     </listitem>
     <listitem>
      <para>
	<function>add_timer()</function> and <function>del_timer()</function>
      </para>
     </listitem>
    </itemizedlist>
   </sect1>
  </chapter>

  <chapter id="apiref">
   <title>Mutex API reference</title>
!Iinclude/linux/mutex.h
!Ekernel/mutex.c
  </chapter>

  <chapter id="references">
   <title>Further reading</title>

   <itemizedlist>
    <listitem>
     <para>
       <filename>Documentation/spinlocks.txt</filename>: 
       Linus Torvalds' spinlocking tutorial in the kernel sources.
     </para>
    </listitem>

    <listitem>
     <para>
       Unix Systems for Modern Architectures: Symmetric
       Multiprocessing and Caching for Kernel Programmers:
     </para>

     <para>
       Curt Schimmel's very good introduction to kernel level
       locking (not written for Linux, but nearly everything
       applies).  The book is expensive, but really worth every
       penny to understand SMP locking. [ISBN: 0201633388]
     </para>
    </listitem>
   </itemizedlist>
  </chapter>

  <chapter id="thanks">
    <title>Thanks</title>

    <para>
      Thanks to Telsa Gwynne for DocBooking, neatening and adding
      style.
    </para>

    <para>
      Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul
      Mackerras, Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim
      Waugh, Pete Zaitcev, James Morris, Robert Love, Paul McKenney,
      John Ashby for proofreading, correcting, flaming, commenting.
    </para>

    <para>
      Thanks to the cabal for having no influence on this document.
    </para>
  </chapter>

  <glossary id="glossary">
   <title>Glossary</title>

   <glossentry id="gloss-preemption">
    <glossterm>preemption</glossterm>
     <glossdef>
      <para>
        Prior to 2.5, or when <symbol>CONFIG_PREEMPT</symbol> is
        unset, processes in user context inside the kernel would not
        preempt each other (ie. you had that CPU until you gave it up,
        except for interrupts).  With the addition of
        <symbol>CONFIG_PREEMPT</symbol> in 2.5.4, this changed: when
        in user context, higher priority tasks can "cut in": spinlocks
        were changed to disable preemption, even on UP.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-bh">
    <glossterm>bh</glossterm>
     <glossdef>
      <para>
        Bottom Half: for historical reasons, functions with
        '_bh' in them often now refer to any software interrupt, e.g.
        <function>spin_lock_bh()</function> blocks any software interrupt 
        on the current CPU.  Bottom halves are deprecated, and will 
        eventually be replaced by tasklets.  Only one bottom half will be 
        running at any time.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-hwinterrupt">
    <glossterm>Hardware Interrupt / Hardware IRQ</glossterm>
    <glossdef>
     <para>
       Hardware interrupt request.  <function>in_irq()</function> returns 
       <returnvalue>true</returnvalue> in a hardware interrupt handler.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-interruptcontext">
    <glossterm>Interrupt Context</glossterm>
    <glossdef>
     <para>
       Not user context: processing a hardware irq or software irq.
       Indicated by the <function>in_interrupt()</function> macro 
       returning <returnvalue>true</returnvalue>.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-smp">
    <glossterm><acronym>SMP</acronym></glossterm>
    <glossdef>
     <para>
       Symmetric Multi-Processor: kernels compiled for multiple-CPU
       machines.  (CONFIG_SMP=y).
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-softirq">
    <glossterm>Software Interrupt / softirq</glossterm>
    <glossdef>
     <para>
       Software interrupt handler.  <function>in_irq()</function> returns
       <returnvalue>false</returnvalue>; <function>in_softirq()</function>
       returns <returnvalue>true</returnvalue>.  Tasklets and softirqs
	both fall into the category of 'software interrupts'.
     </para>
     <para>
       Strictly speaking a softirq is one of up to 32 enumerated software
       interrupts which can run on multiple CPUs at once.
       Sometimes used to refer to tasklets as
       well (ie. all software interrupts).
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-tasklet">
    <glossterm>tasklet</glossterm>
    <glossdef>
     <para>
       A dynamically-registrable software interrupt,
       which is guaranteed to only run on one CPU at a time.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-timers">
    <glossterm>timer</glossterm>
    <glossdef>
     <para>
       A dynamically-registrable software interrupt, which is run at
       (or close to) a given time.  When running, it is just like a
       tasklet (in fact, they are called from the TIMER_SOFTIRQ).
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-up">
    <glossterm><acronym>UP</acronym></glossterm>
    <glossdef>
     <para>
       Uni-Processor: Non-SMP.  (CONFIG_SMP=n).
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-usercontext">
    <glossterm>User Context</glossterm>
    <glossdef>
     <para>
       The kernel executing on behalf of a particular process (ie. a
       system call or trap) or kernel thread.  You can tell which
       process with the <symbol>current</symbol> macro.)  Not to
       be confused with userspace.  Can be interrupted by software or
       hardware interrupts.
     </para>
    </glossdef>
   </glossentry>

   <glossentry id="gloss-userspace">
    <glossterm>Userspace</glossterm>
    <glossdef>
     <para>
       A process executing its own code outside the kernel.
     </para>
    </glossdef>
   </glossentry>      

  </glossary>
</book>