From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51690) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YufDL-0007oK-NO for qemu-devel@nongnu.org; Tue, 19 May 2015 06:57:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YufDI-0001pG-OT for qemu-devel@nongnu.org; Tue, 19 May 2015 06:57:35 -0400 Sender: Paolo Bonzini Message-ID: <555B1718.7010501@redhat.com> Date: Tue, 19 May 2015 12:57:28 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1432036186-29903-1-git-send-email-famz@redhat.com> <1432036186-29903-14-git-send-email-famz@redhat.com> <555AEEA3.4080407@redhat.com> <20150519164816.GA5375@cpc-pc.redhat.com> <555AF925.6050809@redhat.com> <20150519183752.GA2986@cpc-pc.redhat.com> In-Reply-To: <20150519183752.GA2986@cpc-pc.redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [PATCH v4 13/13] block/mirror: Block "device IO" during mirror exit List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng Cc: qemu-block@nongnu.org, "Michael S. Tsirkin" , Juan Quintela , qemu-devel@nongnu.org, mreitz@redhat.com, Stefan Hajnoczi , Amit Shah On 19/05/2015 20:37, Fam Zheng wrote: > On Tue, 05/19 10:49, Paolo Bonzini wrote: >> >> >> On 19/05/2015 18:48, Fam Zheng wrote: >>>>> >>>>> This is too late. As a rule, the blocker must be established before >>>>> calling bdrv_drain, and removed on the next yield (in this case, before >>>>> the assignment to last_pause_ns). >>> I don't understand. If the blocker is removed before mirror_run returns, >>> wouldn't more device IO already hit source image by the time mirror_exit runs? >> >> If you go to mirror_exit, you won't reach the assignment (so you have to >> remove the blocker in mirror_exit too). >> >> But if you don't go to mirror_exit because cnt != 0, you must remove the >> blocker before the next I/O. >> > > OK, but I'm still not clear how is it too late in this patch? Although the > blocker is set after bdrv_drain, we know there is no dirty data because cnt is > 0, and we'll be holding a blocker when releasing the AioContext, no new IO is > allowed. So you rely on the caller of mirror_run not calling aio_context_release between bdrv_drain and block_job_defer_to_main_loop? That indeed should work, but why not stick to a common pattern of blocking I/O before bdrv_drain? That's how bdrv_drain is almost always used in the code, so it's a safe thing to do and the preemption points are then documented more clearly. I think it would be nice to have all bdrv_drain calls: - either preceded by a device I/O blocker - or preceded by a comment explaining why the call is there and why it doesn't need the blocker This is not a NACK, but I would like to understand the disadvantages of what I am suggesting here. Paolo