On Wed, Apr 08, 2015 at 04:56:14PM +0200, Alberto Garcia wrote: > 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().
That is currently not possible since the QEMU monitor does not run while we're waiting in aio_poll(). Therefore no block job monitor commands could spawn a new job. If code is added that spawns a job based on an AioContext timer or due to some other event, then this assumption no longer holds and there is a problem because block_job_resume() is called on a job that never paused. But for now there is no problem. Stefan
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