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

[Avi Kivity <avi@redhat.com>]

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)

Valid:
  lock(find)

Valid:
  lock(coarse)

Invalid:
  lock(fine)
  lock(coarse)


-- 
error compiling committee.c: too many arguments to function