From cbdb5025a3127d20a1e18a14bbcb0cc17be84710 Mon Sep 17 00:00:00 2001
From: Manuel Bentele
Date: Wed, 2 Dec 2020 07:07:44 +0100
Subject: Setup xloop device with XLOOP_CONFIGURE ioctl call

This feature allows the userspace tool xlosetup to completely setup a
loop device with a single ioctl call, removing the in-between state
where the device can be partially configured, eg. the loop device has
a backing file associated with it, but is reading from the wrong offset.

Besides removing the intermediate state, another big benefit of this
ioctl is that XLOOP_SET_STATUS can be slow. The main reason for this
slowness is that XLOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to
freeze the associated queue. This requires waiting for RCU.
---
 src/utils/sys-utils/xlosetup.8 |  7 +++++++
 src/utils/sys-utils/xlosetup.c | 10 +++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

(limited to 'src/utils/sys-utils')

diff --git a/src/utils/sys-utils/xlosetup.8 b/src/utils/sys-utils/xlosetup.8
index 4e063e6..c948899 100644
--- a/src/utils/sys-utils/xlosetup.8
+++ b/src/utils/sys-utils/xlosetup.8
@@ -68,6 +68,13 @@ It's possible to create more independent xloop devices for the same backing
 file.
 .B This setup may be dangerous, can cause data loss, corruption and overwrites.
 Use \fB\-\-nooverlap\fR with \fB\-\-find\fR during setup to avoid this problem.
+.sp
+The xloop device setup is not an atomic operation when used with \fB\-\-find\fP, and
+.B xlosetup
+does not protect this operation by any lock.  The number of attempts is
+internally restricted to a maximum of 64.  It is recommended to use for example
+.BR flock (1)
+to avoid a collision in heavily parallel use cases.
 
 .SH OPTIONS
 The \fIsize\fR and \fIoffset\fR
diff --git a/src/utils/sys-utils/xlosetup.c b/src/utils/sys-utils/xlosetup.c
index 60fa0ba..cbc2e69 100644
--- a/src/utils/sys-utils/xlosetup.c
+++ b/src/utils/sys-utils/xlosetup.c
@@ -469,7 +469,7 @@ static int create_loop(struct loopdev_cxt *lc,
 		       uint64_t blocksize, uint32_t file_fmt_type)
 {
 	int hasdev = loopcxt_has_device(lc);
-	int rc = 0;
+	int rc = 0, ntries = 0;
 
 	/* xlosetup --find --noverlap file.img */
 	if (!hasdev && nooverlap) {
@@ -501,7 +501,7 @@ static int create_loop(struct loopdev_cxt *lc,
 				errx(EXIT_FAILURE, _("%s: overlapping encrypted xloop device exists"), file);
 			}
 
-			lc->info.lo_flags &= ~LO_FLAGS_AUTOCLEAR;
+			lc->config.info.lo_flags &= ~LO_FLAGS_AUTOCLEAR;
 			if (loopcxt_ioctl_status(lc)) {
 				loopcxt_deinit(lc);
 				errx(EXIT_FAILURE, _("%s: failed to re-use xloop device"), file);
@@ -571,8 +571,12 @@ static int create_loop(struct loopdev_cxt *lc,
 		rc = loopcxt_setup_device(lc);
 		if (rc == 0)
 			break;			/* success */
-		if (errno == EBUSY && !hasdev)
+
+		if (errno == EBUSY && !hasdev && ntries < 64) {
+			xusleep(200000);
+			ntries++;
 			continue;
+		}
 
 		/* errors */
 		errpre = hasdev && loopcxt_get_fd(lc) < 0 ?
-- 
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