From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:33475) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T9Z0O-0001xD-SR for qemu-devel@nongnu.org; Thu, 06 Sep 2012 06:08:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1T9Z0C-0000Xs-Pd for qemu-devel@nongnu.org; Thu, 06 Sep 2012 06:08:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13486) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T9Z0C-0000Xi-HF for qemu-devel@nongnu.org; Thu, 06 Sep 2012 06:08:00 -0400 Message-ID: <504875F6.9090302@redhat.com> Date: Thu, 06 Sep 2012 12:07:50 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <20120809130010.GA7960@in.ibm.com> <20120809130216.GC7960@in.ibm.com> <20120905074106.GA28080@in.ibm.com> <20120905095431.GB28080@in.ibm.com> <50484F63.5050406@redhat.com> <504867A9.3050808@redhat.com> <50486F24.2010106@redhat.com> In-Reply-To: <50486F24.2010106@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v6 2/2] block: Support GlusterFS as a QEMU block backend List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Anthony Liguori , Anand Avati , Stefan Hajnoczi , Vijay Bellur , Amar Tumballi , qemu-devel@nongnu.org, Blue Swirl , bharata@linux.vnet.ibm.com Am 06.09.2012 11:38, schrieb Paolo Bonzini: > Il 06/09/2012 11:06, Kevin Wolf ha scritto: >>>> If it works, I think this change would be preferrable to using a "magic" >>>> BH in every driver. >> The way it works in posix-aio-compat is that the request is first >> removed from the list and then the callback is called. This way >> posix_aio_flush() can return 0 and bdrv_drain_all() completes. > > So the same could be done in gluster: first decrease qemu_aio_count, > then call the callback, then call qemu_aio_release. > > But in either case, wouldn't that leak the AIOCBs until the end of > qcow2_create? > > The AIOCB is already invalid at the time the callback is entered, so we > could release it before the call. However, not all implementation of > AIO are ready for that and I'm not really in the mood for large scale > refactoring... But the way, what I'd really want to see in the end is to get rid of qemu_aio_flush() and replace it by .bdrv_drain() callbacks in each BlockDriver. The way we're doing it today is a layering violation. Doesn't change anything about this problem, though. So the options that we have are: 1. Delay the callback using a BH. Doing this in each driver is ugly. But is there actually more than one possible callback in today's coroutine world? I only see bdrv_co_io_em_complete(), which could reenter the coroutine from a BH. 2. Delay the callback by just calling it later when the cleanup has been completed and .io_flush() can return 0. You say that it's hard to implement for some drivers, except if the AIOCB are leaked until the end of functions like qcow2_create(). 3. Add a delay only later in functions like bdrv_drain_all() that assume that the request has completed. Are there more of this type? AIOCBs are leaked until a bdrv_drain_all() call. Does it work with draining specific BDSes instead of everything? Unless I forgot some important point, it almost looks like option 1 is the easiest and safest. Kevin