[RESEND][BUG FIX HELP] QEMU main thread endlessly hangs in __ppoll()

[Like Xu <like.xu@linux.intel.com>]

Hi Genius,

I am a user of QEMU v4.2.0 and stuck in an interesting bug, which may still 
exist in the mainline.
Thanks in advance to heroes who can take a look and share understanding.

The qemu main thread endlessly hangs in the handle of the qmp statement:
{'execute': 'human-monitor-command', 'arguments':{ 'command-line': 
'drive_del replication0' } }
and we have the call trace looks like:

#0 0x00007f3c22045bf6 in __ppoll (fds=0x555611328410, nfds=1, 
timeout=<optimized out>, timeout@entry=0x7ffc56c66db0,
sigmask=sigmask@entry=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:44
#1 0x000055561021f415 in ppoll (__ss=0x0, __timeout=0x7ffc56c66db0, 
__nfds=<optimized out>, __fds=<optimized out>)
at /usr/include/x86_64-linux-gnu/bits/poll2.h:77
#2 qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, 
timeout=<optimized out>) at util/qemu-timer.c:348
#3 0x0000555610221430 in aio_poll (ctx=ctx@entry=0x5556113010f0, 
blocking=blocking@entry=true) at util/aio-posix.c:669
#4 0x000055561019268d in bdrv_do_drained_begin (poll=true, 
ignore_bds_parents=false, parent=0x0, recursive=false,
bs=0x55561138b0a0) at block/io.c:430
#5 bdrv_do_drained_begin (bs=0x55561138b0a0, recursive=<optimized out>, 
parent=0x0, ignore_bds_parents=<optimized out>,
poll=<optimized out>) at block/io.c:396
#6 0x000055561017b60b in quorum_del_child (bs=0x55561138b0a0, 
child=0x7f36dc0ce380, errp=<optimized out>)
at block/quorum.c:1063
#7 0x000055560ff5836b in qmp_x_blockdev_change (parent=0x555612373120 
"colo-disk0", has_child=<optimized out>,
child=0x5556112df3e0 "children.1", has_node=<optimized out>, node=0x0, 
errp=0x7ffc56c66f98) at blockdev.c:4494
#8 0x00005556100f8f57 in qmp_marshal_x_blockdev_change (args=<optimized 
out>, ret=<optimized out>, errp=0x7ffc56c67018)
at qapi/qapi-commands-block-core.c:1538
#9 0x00005556101d8290 in do_qmp_dispatch (errp=0x7ffc56c67010, 
allow_oob=<optimized out>, request=<optimized out>,
cmds=0x5556109c69a0 <qmp_commands>) at qapi/qmp-dispatch.c:132
#10 qmp_dispatch (cmds=0x5556109c69a0 <qmp_commands>, request=<optimized 
out>, allow_oob=<optimized out>)
at qapi/qmp-dispatch.c:175
#11 0x00005556100d4c4d in monitor_qmp_dispatch (mon=0x5556113a6f40, 
req=<optimized out>) at monitor/qmp.c:145
#12 0x00005556100d5437 in monitor_qmp_bh_dispatcher (data=<optimized out>) 
at monitor/qmp.c:234
#13 0x000055561021dbec in aio_bh_call (bh=0x5556112164bGrateful0) at 
util/async.c:117
#14 aio_bh_poll (ctx=ctx@entry=0x5556112151b0) at util/async.c:117
#15 0x00005556102212c4 in aio_dispatch (ctx=0x5556112151b0) at 
util/aio-posix.c:459
#16 0x000055561021dab2 in aio_ctx_dispatch (source=<optimized out>, 
callback=<optimized out>, user_data=<optimized out>)
at util/async.c:260
#17 0x00007f3c22302fbd in g_main_context_dispatch () from 
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#18 0x0000555610220358 in glib_pollfds_poll () at util/main-loop.c:219
#19 os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
#20 main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:518
#21 0x000055560ff600fe in main_loop () at vl.c:1814
#22 0x000055560fddbce9 in main (argc=<optimized out>, argv=<optimized out>, 
envp=<optimized out>) at vl.c:4503

We found that we're doing endless check in the line of 
block/io.c:bdrv_do_drained_begin():
	BDRV_POLL_WHILE(bs, bdrv_drain_poll_top_level(bs, recursive, parent));
and it turns out that the bdrv_drain_poll() always get true from:
- bdrv_parent_drained_poll(bs, ignore_parent, ignore_bds_parents)
- AND atomic_read(&bs->in_flight)

I personally think this is a deadlock issue in the a QEMU block layer
(as we know, we have some #FIXME comments in related codes, such as block 
permisson update).
Any comments are welcome and appreciated.

---
thx,likexu