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Re: [Qemu-devel] [Qemu-block] segfault in parallel blockjobs (iotest 30)

From: Alberto Garcia
Subject: Re: [Qemu-devel] [Qemu-block] segfault in parallel blockjobs (iotest 30)
Date: Wed, 08 Nov 2017 15:45:38 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Tue 07 Nov 2017 05:19:41 PM CET, Anton Nefedov wrote:
> BlockBackend gets deleted by another job's stream_complete(), deferred
> to the main loop, so the fact that the job is put to sleep by
> bdrv_drain_all_begin() doesn't really stop it from execution.

I was debugging this a bit, and the block_job_defer_to_main_loop() call
happens _after_ all jobs have been paused, so I think that when the BDS
is drained then stream_run() finishes the last iteration without
checking if it's paused.

Without your patch (i.e. with a smaller STREAM_BUFFER_SIZE) then I
assume that the function would have to continue looping and
block_job_sleep_ns() would make the job coroutine yield, effectively
pausing the job and preventing the crash.

I can fix the crash by adding block_job_pause_point(&s->common) at the
end of stream_run() (where the 'out' label is).

I'm thinking that perhaps we should add the pause point directly to
block_job_defer_to_main_loop(), to prevent any block job from running
the exit function when it's paused.

Somehow I had the impression that we discussed this already in the past
(?) because I remember thinking about this very scenario.


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