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Date: Tue, 19 May 2015 18:37:52 +0000
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v4 13/13] block/mirror: Block "device IO"
 during mirror exit
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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>

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