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Date: Wed, 19 Sep 2012 11:14:53 +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 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.

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.

Paolo

> Otherwise another thread will take biglock then local lock, which
> cause the possibility of deadlock.
>