From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:45593)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1TEGae-0004E7-MG
	for qemu-devel@nongnu.org; Wed, 19 Sep 2012 05:29:08 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1TEGaY-0000hL-S0
	for qemu-devel@nongnu.org; Wed, 19 Sep 2012 05:29:04 -0400
Received: from david.siemens.de ([192.35.17.14]:22340)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <jan.kiszka@siemens.com>) id 1TEGaY-0000hH-IG
	for qemu-devel@nongnu.org; Wed, 19 Sep 2012 05:28:58 -0400
Message-ID: <50599058.3040904@siemens.com>
Date: Wed, 19 Sep 2012 11:28:56 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
MIME-Version: 1.0
References: <CAJnKYQmv=k9QhcTfVzXcdqnFxZiCsZTehC8pGWeVCfBq7FX0zg@mail.gmail.com>
	<50597D1F.3070607@redhat.com>
	<CAJnKYQ=yKMNLtsQGu3ABvug+XrVr9DbGnKq3DhWE8koLKWw6Aw@mail.gmail.com>
	<50598B58.4010704@redhat.com>
	<CAJnKYQkGkQ+8Y3R7Z9gJmMFhPen9ygD01aMwPxmMoVcGOHws5g@mail.gmail.com>
	<50598D0D.2090608@redhat.com>
	<CAJnKYQk6ZhssCcEV1e_h27YkO4qvqqgTM_QprqCXjcZK2WmikA@mail.gmail.com>
	<50598F0C.2040301@redhat.com> <50598FFA.5@siemens.com>
In-Reply-To: <50598FFA.5@siemens.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [big lock] Discussion about the convention of
 device's DMA each other after breaking down biglock
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, liu ping fan <qemulist@gmail.com>, Avi Kivity <avi@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Paolo Bonzini <pbonzini@redhat.com>

On 2012-09-19 11:27, Jan Kiszka wrote:
> On 2012-09-19 11:23, Avi Kivity wrote:
>> On 09/19/2012 12:19 PM, liu ping fan wrote:
>>> On Wed, Sep 19, 2012 at 5:14 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>> Il 19/09/2012 11:11, liu ping fan ha scritto:
>>>>>>> Why not? devA will drop its local lock, devX will retake the big lock
>>>>>>> recursively, devB will take its local lock.  In the end, we have biglock
>>>>>>> -> devB.
>>>>>>>
>>>>> But when adopting local lock, we assume take local lock, then biglock.
>>>>
>>>> No, because the local lock will be dropped before taking the biglock.
>>>> The order must always be coarse->fine.
>>>>
>>> But if we takes coarse firstly, then the mmio-dispatcher will still
>>> contend for the big lock against each other.
>>
>> Can you detail the sequence?
>>
>>>
>>>> I don't know if the front-end (device) lock should come before or after
>>>> the back-end (e.g. netdev) lock in the hierarchy, but that's another story.
>>>>
>>> I think fine->coarse may be the rule, since for some code path, it is
>>> not necessary to take coarse lock.
>>
>> coarse->fine doesn't mean you have to take the coarse lock.
>>
>> Valid:
>>   lock(coarse)
>>   lock(fine)
> 
> This is invalid due to prio inversion issues, independent of potential
> deadlocks.

Err, forget it - the other way around is the problem.

Sorry,
Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux