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Date: Wed, 19 Sep 2012 11:27:22 +0200
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Subject: Re: [Qemu-devel] [big lock] Discussion about the convention of
 device's DMA each other after breaking down biglock
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To: Avi Kivity <avi@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, liu ping fan <qemulist@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

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.

Jan

> 
> Valid:
>   lock(find)
> 
> Valid:
>   lock(coarse)
> 
> Invalid:
>   lock(fine)
>   lock(coarse)
> 
> 

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