Re: [Qemu-devel] [PATCH v2 5/6] iothread: fix iothread_stop() race condition

[Eric Blake <eblake@redhat.com>]

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On 12/07/2017 02:13 PM, Stefan Hajnoczi wrote:
> There is a small chance that iothread_stop() hangs as follows:
> 

> 
> The bug is explained by the AioContext->notify_me doc comments:
> 
>   "If this field is 0, everything (file descriptors, bottom halves,
>   timers) will be re-evaluated before the next blocking poll(), thus the
>   event_notifier_set call can be skipped."
> 
> The problem is that "everything" does not include checking
> iothread->stopping.  This means iothread_run() will block in aio_poll()
> if aio_notify() was called just before aio_poll().
> 
> This patch fixes the hang by replacing aio_notify() with
> aio_bh_schedule_oneshot().  This makes aio_poll() or g_main_loop_run()
> to return.

s/to //

> 
> Implementing this properly required a new bool running flag.  The new
> flag prevents races that are tricky if we try to use iothread->stopping.
> Now iothread->stopping is purely for iothread_stop() and
> iothread->running is purely for the iothread_run() thread.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> I'm including this patch in this series because it is needed to make the
> test case reliable.  It's unrelated to the main goal of the patch
> series.
> ---
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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