block/backup-top: Don't acquire context while dropping top

All paths that lead to bdrv_backup_top_drop(), except for the call
from backup_clean(), imply that the BDS AioContext has already been
acquired, so doing it there too can potentially lead to QEMU hanging
on AIO_WAIT_WHILE().

An easy way to trigger this situation is by issuing a two actions
transaction, with a proper and a bogus blockdev-backup, so the second
one will trigger a rollback. This will trigger a hang with an stack
trace like this one:

 #0  0x00007fb680c75016 in __GI_ppoll (fds=0x55e74580f7c0, nfds=1, timeout=<optimized out>,
     timeout@entry=0x0, sigmask=sigmask@entry=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:39
 #1  0x000055e743386e09 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>)
     at /usr/include/bits/poll2.h:77
 #2  0x000055e743386e09 in qemu_poll_ns
     (fds=<optimized out>, nfds=<optimized out>, timeout=<optimized out>) at util/qemu-timer.c:336
 #3  0x000055e743388dc4 in aio_poll (ctx=0x55e7458925d0, blocking=blocking@entry=true)
     at util/aio-posix.c:669
 #4  0x000055e743305dea in bdrv_flush (bs=bs@entry=0x55e74593c0d0) at block/io.c:2878
 #5  0x000055e7432be58e in bdrv_close (bs=0x55e74593c0d0) at block.c:4017
 #6  0x000055e7432be58e in bdrv_delete (bs=<optimized out>) at block.c:4262
 #7  0x000055e7432be58e in bdrv_unref (bs=bs@entry=0x55e74593c0d0) at block.c:5644
 #8  0x000055e743316b9b in bdrv_backup_top_drop (bs=bs@entry=0x55e74593c0d0) at block/backup-top.c:273
 #9  0x000055e74331461f in backup_job_create
     (job_id=0x0, bs=bs@entry=0x55e7458d5820, target=target@entry=0x55e74589f640, speed=0, sync_mode=MIRROR_SYNC_MODE_FULL, sync_bitmap=sync_bitmap@entry=0x0, bitmap_mode=BITMAP_SYNC_MODE_ON_SUCCESS, compress=false, filter_node_name=0x0, on_source_error=BLOCKDEV_ON_ERROR_REPORT, on_target_error=BLOCKDEV_ON_ERROR_REPORT, creation_flags=0, cb=0x0, opaque=0x0, txn=0x0, errp=0x7ffddfd1efb0) at block/backup.c:478
 #10 0x000055e74315bc52 in do_backup_common
     (backup=backup@entry=0x55e746c066d0, bs=bs@entry=0x55e7458d5820, target_bs=target_bs@entry=0x55e74589f640, aio_context=aio_context@entry=0x55e7458a91e0, txn=txn@entry=0x0, errp=errp@entry=0x7ffddfd1efb0)
     at blockdev.c:3580
 #11 0x000055e74315c37c in do_blockdev_backup
     (backup=backup@entry=0x55e746c066d0, txn=0x0, errp=errp@entry=0x7ffddfd1efb0)
     at /usr/src/debug/qemu-kvm-4.2.0-2.module+el8.2.0+5135+ed3b2489.x86_64/./qapi/qapi-types-block-core.h:1492
 #12 0x000055e74315c449 in blockdev_backup_prepare (common=0x55e746a8de90, errp=0x7ffddfd1f018)
     at blockdev.c:1885
 #13 0x000055e743160152 in qmp_transaction
     (dev_list=<optimized out>, has_props=<optimized out>, props=0x55e7467fe2c0, errp=errp@entry=0x7ffddfd1f088) at blockdev.c:2340
 #14 0x000055e743287ff5 in qmp_marshal_transaction
     (args=<optimized out>, ret=<optimized out>, errp=0x7ffddfd1f0f8)
     at qapi/qapi-commands-transaction.c:44
 #15 0x000055e74333de6c in do_qmp_dispatch
     (errp=0x7ffddfd1f0f0, allow_oob=<optimized out>, request=<optimized out>, cmds=0x55e743c28d60 <qmp_commands>) at qapi/qmp-dispatch.c:132
 #16 0x000055e74333de6c in qmp_dispatch
     (cmds=0x55e743c28d60 <qmp_commands>, request=<optimized out>, allow_oob=<optimized out>)
     at qapi/qmp-dispatch.c:175
 #17 0x000055e74325c061 in monitor_qmp_dispatch (mon=0x55e745908030, req=<optimized out>)
     at monitor/qmp.c:145
 #18 0x000055e74325c6fa in monitor_qmp_bh_dispatcher (data=<optimized out>) at monitor/qmp.c:234
 #19 0x000055e743385866 in aio_bh_call (bh=0x55e745807ae0) at util/async.c:117
 #20 0x000055e743385866 in aio_bh_poll (ctx=ctx@entry=0x55e7458067a0) at util/async.c:117
 #21 0x000055e743388c54 in aio_dispatch (ctx=0x55e7458067a0) at util/aio-posix.c:459
 #22 0x000055e743385742 in aio_ctx_dispatch
     (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:260
 #23 0x00007fb68543e67d in g_main_dispatch (context=0x55e745893a40) at gmain.c:3176
 #24 0x00007fb68543e67d in g_main_context_dispatch (context=context@entry=0x55e745893a40) at gmain.c:3829
 #25 0x000055e743387d08 in glib_pollfds_poll () at util/main-loop.c:219
 #26 0x000055e743387d08 in os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
 #27 0x000055e743387d08 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:518
 #28 0x000055e74316a3c1 in main_loop () at vl.c:1828
 #29 0x000055e743016a72 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>)
     at vl.c:4504

Fix this by not acquiring the AioContext there, and ensuring all paths
leading to it have it already acquired (backup_clean()).

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1782111
Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/backup-top.c

@@ -255,9 +255,6 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
 void bdrv_backup_top_drop(BlockDriverState *bs)
 {
     BDRVBackupTopState *s = bs->opaque;
-    AioContext *aio_context = bdrv_get_aio_context(bs);
-
-    aio_context_acquire(aio_context);
 
     bdrv_drained_begin(bs);
 
@@ -271,6 +268,4 @@ void bdrv_backup_top_drop(BlockDriverState *bs)
     bdrv_drained_end(bs);
 
     bdrv_unref(bs);
-
-    aio_context_release(aio_context);
 }

block/backup.c

@@ -135,8 +135,11 @@ 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);
     bdrv_backup_top_drop(s->backup_top);
+    aio_context_release(aio_context);
 }
 
 void backup_do_checkpoint(BlockJob *job, Error **errp)