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Date: Mon, 17 Apr 2017 07:21:39 -0400
From: Jeff Cody <jcody@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH for-2.9-rc5 v2] block: Drain
 BH in bdrv_drained_begin
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To: Fam Zheng <famz@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, Kevin Wolf <kwolf@redhat.com>, qemu block <qemu-block@nongnu.org>, qemu-devel <qemu-devel@nongnu.org>, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On Mon, Apr 17, 2017 at 04:27:19PM +0800, Fam Zheng wrote:
> On Fri, 04/14 09:51, Stefan Hajnoczi wrote:
> > On Fri, Apr 14, 2017 at 9:02 AM, Fam Zheng <famz@redhat.com> wrote:
> > > @@ -398,11 +399,15 @@ void bdrv_drain_all(void);
> > >           */                                                \
> > >          assert(!bs_->wakeup);                              \
> > >          bs_->wakeup = true;                                \
> > > -        while ((cond)) {                                   \
> > > -            aio_context_release(ctx_);                     \
> > > -            aio_poll(qemu_get_aio_context(), true);        \
> > > -            aio_context_acquire(ctx_);                     \
> > > -            waited_ = true;                                \
> > > +        while (busy_) {                                    \
> > > +            if ((cond)) {                                  \
> > > +                waited_ = busy_ = true;                    \
> > > +                aio_context_release(ctx_);                 \
> > > +                aio_poll(qemu_get_aio_context(), true);    \
> > > +                aio_context_acquire(ctx_);                 \
> > > +            } else {                                       \
> > > +                busy_ = aio_poll(ctx_, false);             \
> > > +            }                                              \
> > 
> > Wait, I'm confused.  The current thread is not in the BDS AioContext.
> > We're not allowed to call aio_poll(ctx_, false).
> > 
> > Did you mean aio_poll(qemu_get_aio_context(), false) in order to
> > process defer to main loop BHs?
> 
> At this point it's even unclear to me what should be the plan for 2.9.  v1 IMO
> was the least intrusive, but didn't cover bdrv_drain_all_begin. v2 has this
> controversial "aio_poll(ctx_, false)", however its alternative,
> "aio_poll(qemu_get_aio_context(), false)", "introduces" another crash that is
> not seen otherwise.
> 
> What should we do now?
> 
> Fam
>

I think the priority is fixing the regression, which v1 does.  Is there a
regression lurking in the bdrv_drain_all() path?  I've not been able to find
one.

If there is no regressive path through bdrv_drain_all(), my vote would be
the simplest, least intrusive patch that fixes the current regression, and I
think that is v1.  

Then we can fix everything correctly, and more comprehensively, for 2.10.


-Jeff