Re: [Qemu-devel] [PATCH v6 1/3] linux-aio: fix submit aio as a batch

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Nov 26, 2014 at 05:15:44PM +0800, Ming Lei wrote:
> On Wed, Nov 26, 2014 at 12:18 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >>
> >> You mean the abort BH may not have chance to run before its deletion
> >> in the detach callback?
> >
> > Exactly.  Any time you schedule a BH you need to be aware of things that
> > may happen before the BH is invoked.
> >
> >> If so, bdrv_drain_all() from bdrv_set_aio_context() should have
> >> handled the pending BH, right?
> >
> > I'm not sure if it's good to make subtle assumptions like that.  If the
> > code changes they will break.
> 
> IMO, that should be the purpose of bdrv_drain_all(), at least from
> its comment:
> 
>          /* ensure there are no in-flight requests */
> 
> If it changes in future, the general problem has to be considered.
> 
> > Since it is very easy to protect against this case (the code I posted
> > before), it seems worthwhile to be on the safe side.
> 
> Given there hasn't the potential problem in current tree, could you
> agree on merging it first?
> 
> BTW, there isn't sort of handling for 'completion_bh' of linux aio too, :-)

That's incorrect, completion_bh is protected:

void laio_attach_aio_context(void *s_, AioContext *new_context)
{
    struct qemu_laio_state *s = s_;

    s->completion_bh = aio_bh_new(new_context, qemu_laio_completion_bh, s);
    aio_set_event_notifier(new_context, &s->e, qemu_laio_completion_cb);
    ^---- this will reschedule completion_bh if s->e has events pending

I am asking for a one-line if statement to avoid introducing a subtle
assumption that would be a pain to debug in the future.  It's so easy to
add that I'm against merging the patch without this protection.

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