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Date: Thu, 20 Sep 2012 09:54:44 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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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: liu ping fan <qemulist@gmail.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, Avi Kivity <avi@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, qemu-devel@nongnu.org

Il 20/09/2012 09:51, liu ping fan ha scritto:
> Sorry, donot catching your meaning.  Does not "coarse->fine"  mean
> LOCK(coarse)-->LOCK(fine);  .. UNLOCK(fine)-->UNLOCK(coarse) ?

Yes.

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

No, MMIO dispatch has to discard the fine-grained lock before acquiring
the big lock.

If you allow

  lock(fine)
  lock(coarse)

then the (presumably higher-priority) thread that is requesting the
fine-grained lock must wait for the lower-priority thread that holds the
coarse-grained lock.  Then you get priority inversion.

Paolo