From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:39325)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1ZqWhj-0005uF-Fp
	for qemu-devel@nongnu.org; Sun, 25 Oct 2015 21:36:08 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1ZqWhi-0000pS-KF
	for qemu-devel@nongnu.org; Sun, 25 Oct 2015 21:36:07 -0400
Date: Mon, 26 Oct 2015 09:35:56 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20151026013556.GB28904@ad.usersys.redhat.com>
References: <1445569694-2765-1-git-send-email-famz@redhat.com>
	<1445569694-2765-6-git-send-email-famz@redhat.com>
	<20151023151341.GE5977@stefanha-x1.localdomain>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151023151341.GE5977@stefanha-x1.localdomain>
Subject: Re: [Qemu-devel] [PATCH v7 05/10] block: Introduce "drained
	begin/end" API
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: kwolf@redhat.com, pbonzini@redhat.com, jcody@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Fri, 10/23 16:13, Stefan Hajnoczi wrote:
> On Fri, Oct 23, 2015 at 11:08:09AM +0800, Fam Zheng wrote:
> > +void bdrv_drained_begin(BlockDriverState *bs)
> > +{
> > +    if (!bs->quiesce_counter++) {
> > +        aio_disable_external(bdrv_get_aio_context(bs));
> > +    }
> > +    bdrv_drain(bs);
> > +}
> > +
> > +void bdrv_drained_end(BlockDriverState *bs)
> > +{
> > +    assert(bs->quiesce_counter > 0);
> > +    if (--bs->quiesce_counter > 0) {
> > +        return;
> > +    }
> > +    aio_enable_external(bdrv_get_aio_context(bs));
> > +}
> 
> Why is quiesce_counter necessary?  Can't we just rely on AioContext's
> disable_external_cnt?

It was added because bdrv_drain was conditional in a previous version, so yes
it can now be dropped, but we lose the explcitness of the assertion in
bdrv_drained_end. I was thinking that more "assert(!bs->quiesce_counter)" can
be useful in blk_aio_read/write etc..

Fam