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[PATCH] backup: don't acquire aio_context in backup_clean

From: Stefan Reiter
Subject: [PATCH] backup: don't acquire aio_context in backup_clean
Date: Wed, 25 Mar 2020 16:50:55 +0100

backup_clean is only ever called as a handler via job_exit, which
already acquires the job's context. The job's context is guaranteed to
be the same as the one used by backup_top via backup_job_create.

Since the previous logic effectively acquired the lock twice, this
broke cleanup of backups for disks using IO threads, since the BDRV_POLL_WHILE
in bdrv_backup_top_drop -> bdrv_do_drained_begin would only release the lock
once, thus deadlocking with the IO thread.

Signed-off-by: Stefan Reiter <address@hidden>

This is a fix for the issue discussed in this part of the thread:
...not the original problem (core dump) posted by Dietmar.

I've still seen it occasionally hang during a backup abort. I'm trying to figure
out why that happens, stack trace indicates a similar problem with the main
thread hanging at bdrv_do_drained_begin, though I have no clue why as of yet.

 block/backup.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index 7430ca5883..a7a7dcaf4c 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -126,11 +126,7 @@ static void backup_abort(Job *job)
 static void backup_clean(Job *job)
     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
-    AioContext *aio_context = bdrv_get_aio_context(s->backup_top);
-    aio_context_acquire(aio_context);
-    aio_context_release(aio_context);
 void backup_do_checkpoint(BlockJob *job, Error **errp)

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