From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:57537)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <berto@igalia.com>) id 1YfrOw-0000Dd-5b
	for qemu-devel@nongnu.org; Wed, 08 Apr 2015 10:56:26 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <berto@igalia.com>) id 1YfrOs-0007oN-6G
	for qemu-devel@nongnu.org; Wed, 08 Apr 2015 10:56:22 -0400
Date: Wed, 8 Apr 2015 16:56:14 +0200
From: Alberto Garcia <berto@igalia.com>
Message-ID: <20150408145614.GA2140@igalia.com>
References: <1428069921-2957-1-git-send-email-famz@redhat.com>
	<1428069921-2957-3-git-send-email-famz@redhat.com>
	<20150408103752.GH28835@stefanha-thinkpad.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150408103752.GH28835@stefanha-thinkpad.redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 2/4] block: Pause block jobs in
	bdrv_drain_all
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, qemu-devel@nongnu.org, pbonzini@redhat.com

On Wed, Apr 08, 2015 at 11:37:52AM +0100, Stefan Hajnoczi wrote:

> > +    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> > +        AioContext *aio_context = bdrv_get_aio_context(bs);
> > +
> > +        aio_context_acquire(aio_context);
> > +        if (bs->job) {
> > +            block_job_pause(bs->job);
> > +        }
> > +        aio_context_release(aio_context);
> > +    }
> > +
> >      while (busy) {
> >          busy = false;
> >  
> > @@ -2044,6 +2054,16 @@ void bdrv_drain_all(void)
> >              aio_context_release(aio_context);
> >          }
> >      }
> > +
> > +    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> > +        AioContext *aio_context = bdrv_get_aio_context(bs);
> > +
> > +        aio_context_acquire(aio_context);
> > +        if (bs->job) {
> > +            block_job_resume(bs->job);
> > +        }
> > +        aio_context_release(aio_context);
> > +    }
> >  }
> 
> There is a tiny chance that we pause a job (which actually just sets
> job->paused = true but there's no guarantee the job's coroutine
> reacts to this) right before it terminates.  Then aio_poll() enters
> the coroutine one last time and the job terminates.
> 
> We then reach the resume portion of bdrv_drain_all() and the job no
> longer exists.  Hopefully nothing started a new job in the meantime.
> bs->job should either be the original block job or NULL.
> 
> This code seems under current assumptions, but I just wanted to
> raise these issues in case someone sees problems that I've missed.

Is it possible that a new job is started in the meantime? If that's
the case this will hit the assertion in block_job_resume().

Berto