Re: [Qemu-devel] [PATCH v2 2/4] block: Pause block jobs in bdrv_drain_all

[Alberto Garcia <berto@igalia.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