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Re: bdrv_drained_begin deadlock with io-threads

From: Dietmar Maurer
Subject: Re: bdrv_drained_begin deadlock with io-threads
Date: Tue, 31 Mar 2020 18:18:18 +0200 (CEST)

> On March 31, 2020 5:37 PM Kevin Wolf <address@hidden> wrote:
> Am 31.03.2020 um 17:24 hat Dietmar Maurer geschrieben:
> > 
> > > > How can I see/debug those waiting request?
> > > 
> > > Examine bs->tracked_requests list.
> > > 
> > > BdrvTrackedRequest has "Coroutine *co" field. It's a pointer of coroutine 
> > > of this request. You may use qemu-gdb script to print request's coroutine 
> > > back-trace:
> > 
> > I would, but there are no tracked request at all.
> > 
> > print bs->tracked_requests
> > $2 = {lh_first = 0x0}
> Then it's one of the recursively checked parents, as you already figured
> out.
> > Looks bdrv_parent_drained_poll_single() calls
> > blk_root_drained_poll(), which return true in my case (in_flight > 5).
> Can you identify which BlockBackend is this? Specifically if it's the
> one attached to a guest device or whether it belongs to the block job.

This can trigger from various different places, but the simplest case is when
its called from drive_backup_prepare 

>   bdrv_drained_begin(bs);

which is the backup source drive.

Note: I have added printf debug statement to the source, so the line numbers 
may not exactly match

#0  0x00007ffff5cb3916 in __GI_ppoll (fds=0x7fff63d30c40, nfds=2, 
timeout=<optimized out>, timeout@entry=0x0, sigmask=sigmask@entry=0x0)
    at ../sysdeps/unix/sysv/linux/ppoll.c:39
#1  0x0000555555c60479 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized 
out>, __fds=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/poll2.h:77
#2  0x0000555555c60479 in qemu_poll_ns (fds=<optimized out>, nfds=<optimized 
out>, timeout=timeout@entry=-1) at util/qemu-timer.c:335
#3  0x0000555555c62c61 in fdmon_poll_wait (ctx=0x7fffe8905e80, 
ready_list=0x7fffffffda68, timeout=-1) at util/fdmon-poll.c:79
#4  0x0000555555c62247 in aio_poll (ctx=0x7fffe8905e80, 
blocking=blocking@entry=true) at util/aio-posix.c:589
#5  0x0000555555bc25d5 in bdrv_do_drained_begin (poll=<optimized out>, 
ignore_bds_parents=false, parent=0x0, recursive=false, bs=0x7fffe8954bc0)
    at block/io.c:430
#6  0x0000555555bc25d5 in bdrv_do_drained_begin
    (bs=0x7fffe8954bc0, recursive=<optimized out>, parent=0x0, 
ignore_bds_parents=<optimized out>, poll=<optimized out>) at block/io.c:395
#7  0x000055555595f753 in drive_backup_prepare (common=0x7fffe560c200, 
errp=0x7fffffffdba8) at blockdev.c:1759
#8  0x0000555555961f0a in qmp_transaction 
(dev_list=dev_list@entry=0x7fffffffdc20, has_props=has_props@entry=false, 
    props@entry=0x0, errp=errp@entry=0x7fffffffdc58) at blockdev.c:2406
#9  0x00005555559633c2 in blockdev_do_action (errp=0x7fffffffdc58, 
action=0x7fffffffdc10) at blockdev.c:1054
#10 0x00005555559633c2 in qmp_drive_backup (backup=backup@entry=0x7fffffffdc60, 
errp=errp@entry=0x7fffffffdc58) at blockdev.c:3134
#11 0x0000555555b1e1d1 in qmp_marshal_drive_backup (args=<optimized out>, 
ret=<optimized out>, errp=0x7fffffffdd38) at qapi/qapi-commands-block-core.c:555
#12 0x0000555555c15b08 in qmp_dispatch (cmds=0x555556162000 <qmp_commands>, 
request=<optimized out>, allow_oob=<optimized out>) at qapi/qmp-dispatch.c:155
#13 0x0000555555affca1 in monitor_qmp_dispatch (mon=0x7fffe8915d80, 
req=<optimized out>) at monitor/qmp.c:145
#14 0x0000555555b0048a in monitor_qmp_bh_dispatcher (data=<optimized out>) at 
#15 0x0000555555c5e9d5 in aio_bh_call (bh=0x7fffe8e8f960) at util/async.c:164
#16 0x0000555555c5e9d5 in aio_bh_poll (ctx=ctx@entry=0x7fffe8f3ef80) at 
#17 0x0000555555c6208e in aio_dispatch (ctx=0x7fffe8f3ef80) at 
#18 0x0000555555c5e8be in aio_ctx_dispatch (source=<optimized out>, 
callback=<optimized out>, user_data=<optimized out>) at util/async.c:298
#19 0x00007ffff7c7ef2e in g_main_context_dispatch () at 
#20 0x0000555555c612f8 in glib_pollfds_poll () at util/main-loop.c:219
#21 0x0000555555c612f8 in os_host_main_loop_wait (timeout=<optimized out>) at 
#22 0x0000555555c612f8 in main_loop_wait (nonblocking=nonblocking@entry=0) at 
#23 0x00005555558fc5a9 in qemu_main_loop () at 
#24 0x0000555555800c3e in main (argc=<optimized out>, argv=<optimized out>, 
envp=<optimized out>) at /home/dietmar/pve5-devel/mirror_qemu/softmmu/main.c:49

> Maybe have a look at the job coroutine, too. You can probably easiest
> find it in the 'jobs' list, and then print the coroutine backtrace for
> job->co.

There is in drive_backup_prepare(), before the job gets created.

> > Looks like I am loosing poll events somewhere?
> I don't think we've lost any event if in_flight > 0. It means that
> something is still supposedly active. Maybe the job deadlocked.

This is a simple call to bdrv_drained_begin(bs) (before we setup the job).

I really nobody else able to reproduce this (somebody already tried to 

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