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Re: backup transaction with io-thread core dumps

From: Dietmar Maurer
Subject: Re: backup transaction with io-thread core dumps
Date: Thu, 26 Mar 2020 10:23:54 +0100 (CET)

> > > As mentioned earlier, even a totally simple/normal backup job fails when
> > > using io-threads and the VM is under load. It results in a total
> > > VM freeze!
> > > 
> > 
> > This is definitely a different issue. I'll take a look at it today.
> Thanks. Stefan found a way to avoid that bug with:
> https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg07749.html
> But there are doubts that this is the correct way to fix it ...

And I just run into another freeze (with Stefans path applied). This time
when I cancel a running backup:

#0  0x00007ffff5cb3916 in __GI_ppoll (fds=0x7fff63d35c40, nfds=2, 
timeout=<optimized out>, timeout@entry=0x0, sigmask=sigmask@entry=0x0) at 
#1  0x0000555555c5fcd9 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized 
out>, __fds=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/poll2.h:77
#2  0x0000555555c5fcd9 in qemu_poll_ns (fds=<optimized out>, nfds=<optimized 
out>, timeout=timeout@entry=-1) at util/qemu-timer.c:335
#3  0x0000555555c624c1 in fdmon_poll_wait (ctx=0x7fffe8905e80, 
ready_list=0x7fffffffd2a8, timeout=-1) at util/fdmon-poll.c:79
#4  0x0000555555c61aa7 in aio_poll (ctx=0x7fffe8905e80, 
blocking=blocking@entry=true) at util/aio-posix.c:589
#5  0x0000555555bc2c83 in bdrv_do_drained_begin (poll=<optimized out>, 
ignore_bds_parents=false, parent=0x0, recursive=false, bs=0x7fffe8954bc0) at 
#6  0x0000555555bc2c83 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  0x0000555555bb3c37 in blk_drain (blk=0x7fffe8ebcf80) at 
#8  0x0000555555bb481d in blk_unref (blk=0x7fffe8ebcf80) at 
#9  0x0000555555b6c835 in block_job_free (job=0x7fff64505000) at blockjob.c:89
#10 0x0000555555b6dd19 in job_unref (job=0x7fff64505000) at job.c:360
#11 0x0000555555b6dd19 in job_unref (job=0x7fff64505000) at job.c:352
#12 0x0000555555b6e69a in job_finish_sync (job=job@entry=0x7fff64505000, 
finish=finish@entry=0x555555b6ec80 <job_cancel_err>, errp=errp@entry=0x0) at 
#13 0x0000555555b6ec9e in job_cancel_sync (job=job@entry=0x7fff64505000) at 

Any ideas?

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