summaryrefslogtreecommitdiffstats
path: root/async.c
blob: 57ac3a818032811ecccf2e3adbec63b4afc71596 (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
/*
 * QEMU System Emulator
 *
 * Copyright (c) 2003-2008 Fabrice Bellard
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "qemu-common.h"
#include "qemu-aio.h"

/*
 * An AsyncContext protects the callbacks of AIO requests and Bottom Halves
 * against interfering with each other. A typical example is qcow2 that accepts
 * asynchronous requests, but relies for manipulation of its metadata on
 * synchronous bdrv_read/write that doesn't trigger any callbacks.
 *
 * However, these functions are often emulated using AIO which means that AIO
 * callbacks must be run - but at the same time we must not run callbacks of
 * other requests as they might start to modify metadata and corrupt the
 * internal state of the caller of bdrv_read/write.
 *
 * To achieve the desired semantics we switch into a new AsyncContext.
 * Callbacks must only be run if they belong to the current AsyncContext.
 * Otherwise they need to be queued until their own context is active again.
 * This is how you can make qemu_aio_wait() wait only for your own callbacks.
 *
 * The AsyncContexts form a stack. When you leave a AsyncContexts, you always
 * return to the old ("parent") context.
 */
struct AsyncContext {
    /* Consecutive number of the AsyncContext (position in the stack) */
    int id;

    /* Anchor of the list of Bottom Halves belonging to the context */
    struct QEMUBH *first_bh;

    /* Link to parent context */
    struct AsyncContext *parent;
};

/* The currently active AsyncContext */
static struct AsyncContext *async_context = &(struct AsyncContext) { 0 };

/*
 * Enter a new AsyncContext. Already scheduled Bottom Halves and AIO callbacks
 * won't be called until this context is left again.
 */
void async_context_push(void)
{
    struct AsyncContext *new = qemu_mallocz(sizeof(*new));
    new->parent = async_context;
    new->id = async_context->id + 1;
    async_context = new;
}

/* Run queued AIO completions and destroy Bottom Half */
static void bh_run_aio_completions(void *opaque)
{
    QEMUBH **bh = opaque;
    qemu_bh_delete(*bh);
    qemu_free(bh);
    qemu_aio_process_queue();
}
/*
 * Leave the currently active AsyncContext. All Bottom Halves belonging to the
 * old context are executed before changing the context.
 */
void async_context_pop(void)
{
    struct AsyncContext *old = async_context;
    QEMUBH **bh;

    /* Flush the bottom halves, we don't want to lose them */
    while (qemu_bh_poll());

    /* Switch back to the parent context */
    async_context = async_context->parent;
    qemu_free(old);

    if (async_context == NULL) {
        abort();
    }

    /* Schedule BH to run any queued AIO completions as soon as possible */
    bh = qemu_malloc(sizeof(*bh));
    *bh = qemu_bh_new(bh_run_aio_completions, bh);
    qemu_bh_schedule(*bh);
}

/*
 * Returns the ID of the currently active AsyncContext
 */
int get_async_context_id(void)
{
    return async_context->id;
}

/***********************************************************/
/* bottom halves (can be seen as timers which expire ASAP) */

struct QEMUBH {
    QEMUBHFunc *cb;
    void *opaque;
    int scheduled;
    int idle;
    int deleted;
    QEMUBH *next;
};

QEMUBH *qemu_bh_new(QEMUBHFunc *cb, void *opaque)
{
    QEMUBH *bh;
    bh = qemu_mallocz(sizeof(QEMUBH));
    bh->cb = cb;
    bh->opaque = opaque;
    bh->next = async_context->first_bh;
    async_context->first_bh = bh;
    return bh;
}

int qemu_bh_poll(void)
{
    QEMUBH *bh, **bhp;
    int ret;

    ret = 0;
    for (bh = async_context->first_bh; bh; bh = bh->next) {
        if (!bh->deleted && bh->scheduled) {
            bh->scheduled = 0;
            if (!bh->idle)
                ret = 1;
            bh->idle = 0;
            bh->cb(bh->opaque);
        }
    }

    /* remove deleted bhs */
    bhp = &async_context->first_bh;
    while (*bhp) {
        bh = *bhp;
        if (bh->deleted) {
            *bhp = bh->next;
            qemu_free(bh);
        } else
            bhp = &bh->next;
    }

    return ret;
}

void qemu_bh_schedule_idle(QEMUBH *bh)
{
    if (bh->scheduled)
        return;
    bh->scheduled = 1;
    bh->idle = 1;
}

void qemu_bh_schedule(QEMUBH *bh)
{
    if (bh->scheduled)
        return;
    bh->scheduled = 1;
    bh->idle = 0;
    /* stop the currently executing CPU to execute the BH ASAP */
    qemu_notify_event();
}

void qemu_bh_cancel(QEMUBH *bh)
{
    bh->scheduled = 0;
}

void qemu_bh_delete(QEMUBH *bh)
{
    bh->scheduled = 0;
    bh->deleted = 1;
}

void qemu_bh_update_timeout(int *timeout)
{
    QEMUBH *bh;

    for (bh = async_context->first_bh; bh; bh = bh->next) {
        if (!bh->deleted && bh->scheduled) {
            if (bh->idle) {
                /* idle bottom halves will be polled at least
                 * every 10ms */
                *timeout = MIN(10, *timeout);
            } else {
                /* non-idle bottom halves will be executed
                 * immediately */
                *timeout = 0;
                break;
            }
        }
    }
}