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From: Stefan Hajnoczi <stefanha@gmail.com>
Date: Fri, 14 Apr 2017 09:51:44 +0100
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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: qemu-devel <qemu-devel@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, qemu block <qemu-block@nongnu.org>, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

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?

Stefan