From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:52449)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XReUf-0004Wx-UO
	for qemu-devel@nongnu.org; Wed, 10 Sep 2014 05:47:24 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XReUZ-0002Qn-P8
	for qemu-devel@nongnu.org; Wed, 10 Sep 2014 05:47:17 -0400
Received: from mx1.redhat.com ([209.132.183.28]:51590)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1XReUZ-0002Qf-I6
	for qemu-devel@nongnu.org; Wed, 10 Sep 2014 05:47:11 -0400
Message-ID: <54101E0F.3040900@redhat.com>
Date: Wed, 10 Sep 2014 11:46:55 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1410328814-12560-1-git-send-email-famz@redhat.com>
	<1410328814-12560-6-git-send-email-famz@redhat.com>
	<541009D3.3070301@redhat.com>
	<20140910093604.GB5620@fam-t430.nay.redhat.com>
In-Reply-To: <20140910093604.GB5620@fam-t430.nay.redhat.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v5 05/22] block: Convert
 bdrv_em_aiocb_info.cancel to .cancel_async
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: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Benoit Canet <benoit@irqsave.net>, Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Liu Yuan <namei.unix@gmail.com>

Il 10/09/2014 11:36, Fam Zheng ha scritto:
>> > 
>> > This could call the callback before I/O is finished.  I/O can then
>> > complete and write to disk stuff that was not meant to be written.
> I think the request is already completed when bdrv_aio_rw_vector returns this
> blockacb. I shouldn't override the return code anyway, but perhaps a nop
> bdrv_aio_cancel_em is better.

Note that the legacy bdrv_read/bdrv_write function calls actually are
AIO-friendly (they run in a coroutine, and can yield).

> > I think there is a pre-existing bug, which should be fixed with a "bool
> > *done" member similar to BlockDriverAIOCBCoroutine's.  But for the sake
> > of conversion to async cancellation, you can just empty bdrv_aio_cancel_em.
> 
> BTW, why is it "bool *done" instead of just "bool done"?

Because, until your patches to add reference counting, this would have
caused a dangling pointer in bdrv_aio_co_cancel_em:

    acb->done = true;
    qemu_bh_delete(acb->bh);
    qemu_aio_release(acb);

instead, using "bool *done" works because bdrv_co_em_bh writes into the
variable of bdrv_aio_co_cancel_em.  This assumes that bdrv_aio_cancel is
only called once (no reentrancy).

Paolo