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Date: Tue, 08 Jul 2014 21:50:29 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] another locking issue in current dataplane code?
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To: Christian Borntraeger <borntraeger@de.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>, Kevin Wolf <kwolf@redhat.com>, ming.lei@canonical.com, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Dominik Dingel <dingel@linux.vnet.ibm.com>

Il 08/07/2014 21:07, Christian Borntraeger ha scritto:
> On 08/07/14 19:08, Paolo Bonzini wrote:
>> Il 08/07/2014 17:59, Stefan Hajnoczi ha scritto:
>>> I sent Christian an initial patch to fix this but now both threads are
>>> stuck in rfifolock_lock() inside cond wait.  That's very strange and
>>> should never happen.
>>
>> I had this patch pending for 2.2:
>>
>> commit 6c81e31615c3cda5ea981a998ba8b1b8ed17de6f
>> Author: Paolo Bonzini <pbonzini@redhat.com>
>> Date:   Mon Jul 7 10:39:49 2014 +0200
>>
>>     iothread: do not rely on aio_poll(ctx, true) result to end a loop
>>
>>     Currently, whenever aio_poll(ctx, true) has completed all pending
>>     work it returns true *and* the next call to aio_poll(ctx, true)
>>     will not block.
>>
>>     This invariant has its roots in qemu_aio_flush()'s implementation
>>     as "while (qemu_aio_wait()) {}".  However, qemu_aio_flush() does
>>     not exist anymore and bdrv_drain_all() is implemented differently;
>>     and this invariant is complicated to maintain and subtly different
>>     from the return value of GMainLoop's g_main_context_iteration.
>>
>>     All calls to aio_poll(ctx, true) except one are guarded by a
>>     while() loop checking for a request to be incomplete, or a
>>     BlockDriverState to be idle.  Modify that one exception in
>>     iothread.c.
>>
>>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> The hangs are gone. Looks like 2.1 material now...
>
> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>

Great, I'll send it out tomorrow morning.

Paolo