Re: [Qemu-devel] [PATCH v3 01/17] block: stop relying on io_flush() in bdrv_drain_all()

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Jun 13, 2013 at 10:33:58AM -0400, Paolo Bonzini wrote:
> Il 10/06/2013 10:38, Stefan Hajnoczi ha scritto:
> > On Mon, Jun 10, 2013 at 02:25:57PM +0200, Stefan Hajnoczi wrote:
> >> @@ -1427,26 +1456,18 @@ void bdrv_close_all(void)
> >>  void bdrv_drain_all(void)
> >>  {
> >>      BlockDriverState *bs;
> >> -    bool busy;
> >> -
> >> -    do {
> >> -        busy = qemu_aio_wait();
> >>  
> >> +    while (bdrv_requests_pending_all()) {
> >>          /* FIXME: We do not have timer support here, so this is effectively
> >>           * a busy wait.
> >>           */
> >>          QTAILQ_FOREACH(bs, &bdrv_states, list) {
> >>              if (!qemu_co_queue_empty(&bs->throttled_reqs)) {
> >>                  qemu_co_queue_restart_all(&bs->throttled_reqs);
> >> -                busy = true;
> >>              }
> >>          }
> >> -    } while (busy);
> >>  
> >> -    /* If requests are still pending there is a bug somewhere */
> >> -    QTAILQ_FOREACH(bs, &bdrv_states, list) {
> >> -        assert(QLIST_EMPTY(&bs->tracked_requests));
> >> -        assert(qemu_co_queue_empty(&bs->throttled_reqs));
> >> +        qemu_aio_wait();
> >>      }
> >>  }
> > 
> > tests/ide-test found an issue here: block.c invokes callbacks from a BH
> > so we may not yet have completed the request when this loop terminates.
> > 
> > Kevin: can you fold in this patch?
> > 
> > diff --git a/block.c b/block.c
> > index 31f7231..e176215 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -1469,6 +1469,9 @@ void bdrv_drain_all(void)
> >  
> >          qemu_aio_wait();
> >      }
> > +
> > +    /* Process pending completion BHs */
> > +    aio_poll(qemu_get_aio_context(), false);
> >  }
> 
> aio_poll could require multiple iterations, this is why the old code was
> starting with "busy = qemu_aio_wait()".  You can add a while loop around
> the new call, or do something more similar to the old code, along the
> lines of "do { QTAILQ_FOREACH(...) ...; busy =
> bdrv_request_pending_all(); busy |= aio_poll(qemu_get_aio_context(),
> busy); } while(busy);".

Good idea, the trick is to use busy as the blocking flag.

I will resend with the fix to avoid making this email thread too messy.

Stefan