Re: [Qemu-devel] [PATCH v4 13/13] block/mirror: Block "device IO" during mirror exit

From: Fam Zheng <famz@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-block@nongnu.org, Juan Quintela <quintela@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	qemu-devel@nongnu.org, mreitz@redhat.com,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Amit Shah <amit.shah@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v4 13/13] block/mirror: Block "device IO" during mirror exit
Date: Tue, 19 May 2015 18:37:52 +0000	[thread overview]
Message-ID: <20150519183752.GA2986@cpc-pc.redhat.com> (raw)
In-Reply-To: <555AF925.6050809@redhat.com>

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.

Fam