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Date: Fri, 1 Apr 2016 18:09:58 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] block: Fix bdrv_drain in coroutine
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, lvivier@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Fri, 04/01 11:49, Paolo Bonzini wrote:
> 
> 
> On 01/04/2016 11:46, Fam Zheng wrote:
> > +
> > +static void bdrv_co_drain_bh_cb(void *opaque)
> > +{
> > +    BdrvCoDrainData *data = opaque;
> > +    Coroutine *co = data->co;
> > +
> > +    bdrv_drain(data->bs);
> > +    data->done = true;
> > +    qemu_coroutine_enter(co, NULL);
> > +}
> > +
> > +static void coroutine_fn bdrv_co_drain(BlockDriverState *bs)
> > +{
> > +    QEMUBH *bh;
> > +    BdrvCoDrainData data;
> > +
> > +    assert(qemu_in_coroutine());
> > +    data = (BdrvCoDrainData) {
> > +        .co = qemu_coroutine_self(),
> > +        .bs = bs,
> > +        .done = false,
> > +    };
> > +    bh = aio_bh_new(bdrv_get_aio_context(bs), bdrv_co_drain_bh_cb, &data),
> > +    qemu_bh_schedule(bh);
> > +
> > +    do {
> > +        qemu_coroutine_yield();
> > +    } while (!data.done);
> 
> The loop and "done" is not necessary.  Also,

I was trying to protect against the bugs similar to the one fixed in
e424aff5f30, but you're right we can make this an assertion and the loop is not
needed. If a calling coroutine is resumed unexpectedly, we will catch it and
fix it.

> 
> > +    qemu_bh_delete(bh);
> 
> this can be moved to bdrv_co_drain_bh_cb before bdrv_drain, so that the
> bottom half doesn't slow down the event loop until bdrv_drain completes.

Good point! Will fix.

Thanks,

Fam