Max reported the following bug:

$ ./qemu-img create -f raw src.img 1G
$ ./qemu-img create -f raw dst.img 1G

$ (echo '
   {"execute":"qmp_capabilities"}
   {"execute":"blockdev-mirror",
    "arguments":{"job-id":"mirror",
                 "device":"source",
                 "target":"target",
                 "sync":"full",
                 "filter-node-name":"mirror-top"}}
'; sleep 3; echo '
   {"execute":"human-monitor-command",
    "arguments":{"command-line":
                 "qemu-io mirror-top \"write 0 1G\""}}') \
| x86_64-softmmu/qemu-system-x86_64 \
   -qmp stdio \
   -blockdev file,node-name=source,filename=src.img \
   -blockdev file,node-name=target,filename=dst.img \
   -object iothread,id=iothr0 \
   -device virtio-blk,drive=source,iothread=iothr0

crashes:

0  raise () at /usr/lib/libc.so.6
1  abort () at /usr/lib/libc.so.6
2  error_exit
   (err=<optimized out>,
   msg=msg@entry=0x55fbb1634790 <__func__.27> "qemu_mutex_unlock_impl")
   at ../util/qemu-thread-posix.c:37
3  qemu_mutex_unlock_impl
   (mutex=mutex@entry=0x55fbb25ab6e0,
   file=file@entry=0x55fbb1636957 "../util/async.c",
   line=line@entry=650)
   at ../util/qemu-thread-posix.c:109
4  aio_context_release (ctx=ctx@entry=0x55fbb25ab680) at ../util/async.c:650
5  bdrv_do_drained_begin
   (bs=bs@entry=0x55fbb3a87000, recursive=recursive@entry=false,
   parent=parent@entry=0x0,
   ignore_bds_parents=ignore_bds_parents@entry=false,
   poll=poll@entry=true) at ../block/io.c:441
6  bdrv_do_drained_begin
   (poll=true, ignore_bds_parents=false, parent=0x0, recursive=false,
   bs=0x55fbb3a87000) at ../block/io.c:448
7  blk_drain (blk=0x55fbb26c5a00) at ../block/block-backend.c:1718
8  blk_unref (blk=0x55fbb26c5a00) at ../block/block-backend.c:498
9  blk_unref (blk=0x55fbb26c5a00) at ../block/block-backend.c:491
10 hmp_qemu_io (mon=0x7fffaf3fc7d0, qdict=<optimized out>)
   at ../block/monitor/block-hmp-cmds.c:628

man pthread_mutex_unlock
...
    EPERM  The  mutex type is PTHREAD_MUTEX_ERRORCHECK or
    PTHREAD_MUTEX_RECURSIVE, or the mutex is a robust mutex, and the
    current thread does not own the mutex.

So, thread doesn't own the mutex. And we have iothread here.

Next, note that AIO_WAIT_WHILE() documents that ctx must be acquired
exactly once by caller. But where is it acquired in the call stack?
Seems nowhere.

qemuio_command do acquire aio context.. But we need context acquired
around blk_unref() as well and actually around blk_insert_bs() too.

Let's refactor qemuio_command so that it doesn't acquire aio context
but callers do that instead. This way we can cleanly acquire aio
context in hmp_qemu_io() around all three calls.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210423134233.51495-1-vsementsov@virtuozzo.com>
[mreitz: Fixed comment]
Signed-off-by: Max Reitz <mreitz@redhat.com>