Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock

[liu ping fan]

On Wed, Sep 19, 2012 at 5:23 PM, Avi Kivity <address@hidden> wrote:
> On 09/19/2012 12:19 PM, liu ping fan wrote:
>> On Wed, Sep 19, 2012 at 5:14 PM, Paolo Bonzini <address@hidden> 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?
>
LOCK(local lock)
.......................
LOCK(big lock)
Access timer/block/network subsystem
UNLOCK(big lock)
.....................
UNLOCK(local lock)
>>
>>> 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.
>
Sorry, donot catching your meaning.  Does not "coarse->fine"  mean
LOCK(coarse)-->LOCK(fine);  .. UNLOCK(fine)-->UNLOCK(coarse) ?

> Valid:
>   lock(coarse)
>   lock(fine)
>
But it is conflict with " localLock(fine) --> bigLock(coarse)" which
is taken when mmio dispatch.

Regards,
pingfan

> Valid:
>   lock(find)
>
> Valid:
>   lock(coarse)
>
> Invalid:
>   lock(fine)
>   lock(coarse)
>
>
> --
> error compiling committee.c: too many arguments to function