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a77fd4bb29
Using the nested aio_poll() in coroutine is a bad idea. This patch
replaces the aio_poll loop in bdrv_drain with a BH, if called in
coroutine.
For example, the bdrv_drain() in mirror.c can hang when a guest issued
request is pending on it in qemu_co_mutex_lock().
Mirror coroutine in this case has just finished a request, and the block
job is about to complete. It calls bdrv_drain() which waits for the
other coroutine to complete. The other coroutine is a scsi-disk request.
The deadlock happens when the latter is in turn pending on the former to
yield/terminate, in qemu_co_mutex_lock(). The state flow is as below
(assuming a qcow2 image):
mirror coroutine scsi-disk coroutine
-------------------------------------------------------------
do last write
qcow2:qemu_co_mutex_lock()
...
scsi disk read
tracked request begin
qcow2:qemu_co_mutex_lock.enter
qcow2:qemu_co_mutex_unlock()
bdrv_drain
while (has tracked request)
aio_poll()
In the scsi-disk coroutine, the qemu_co_mutex_lock() will never return
because the mirror coroutine is blocked in the aio_poll(blocking=true).
With this patch, the added qemu_coroutine_yield() allows the scsi-disk
coroutine to make progress as expected:
mirror coroutine scsi-disk coroutine
-------------------------------------------------------------
do last write
qcow2:qemu_co_mutex_lock()
...
scsi disk read
tracked request begin
qcow2:qemu_co_mutex_lock.enter
qcow2:qemu_co_mutex_unlock()
bdrv_drain.enter
> schedule BH
> qemu_coroutine_yield()
> qcow2:qemu_co_mutex_lock.return
> ...
tracked request end
...
(resumed from BH callback)
bdrv_drain.return
...
Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1459855253-5378-2-git-send-email-famz@redhat.com
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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|---|---|---|
| .. | ||
| accounting.c | ||
| archipelago.c | ||
| backup.c | ||
| blkdebug.c | ||
| blkreplay.c | ||
| blkverify.c | ||
| block-backend.c | ||
| bochs.c | ||
| cloop.c | ||
| commit.c | ||
| crypto.c | ||
| curl.c | ||
| dirty-bitmap.c | ||
| dmg.c | ||
| gluster.c | ||
| io.c | ||
| iscsi.c | ||
| linux-aio.c | ||
| Makefile.objs | ||
| mirror.c | ||
| nbd-client.c | ||
| nbd-client.h | ||
| nbd.c | ||
| nfs.c | ||
| null.c | ||
| parallels.c | ||
| qapi.c | ||
| qcow.c | ||
| qcow2-cache.c | ||
| qcow2-cluster.c | ||
| qcow2-refcount.c | ||
| qcow2-snapshot.c | ||
| qcow2.c | ||
| qcow2.h | ||
| qed-check.c | ||
| qed-cluster.c | ||
| qed-gencb.c | ||
| qed-l2-cache.c | ||
| qed-table.c | ||
| qed.c | ||
| qed.h | ||
| quorum.c | ||
| raw-aio.h | ||
| raw-posix.c | ||
| raw-win32.c | ||
| raw_bsd.c | ||
| rbd.c | ||
| sheepdog.c | ||
| snapshot.c | ||
| ssh.c | ||
| stream.c | ||
| throttle-groups.c | ||
| vdi.c | ||
| vhdx-endian.c | ||
| vhdx-log.c | ||
| vhdx.c | ||
| vhdx.h | ||
| vmdk.c | ||
| vpc.c | ||
| vvfat.c | ||
| win32-aio.c | ||
| write-threshold.c | ||