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Date: Mon, 29 Jul 2013 13:21:40 +0200
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Subject: Re: [Qemu-devel] [RFC v2 3/5] timer: make qemu_clock_enable sync
 between disable and timer's cb
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To: liu ping fan <qemulist@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Alex Bligh <alex@alex.org.uk>, Anthony Liguori <anthony@codemonkey.ws>

Il 29/07/2013 10:10, liu ping fan ha scritto:
> On Mon, Jul 29, 2013 at 2:30 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> Il 29/07/2013 05:16, Liu Ping Fan ha scritto:
>>> After disabling the QemuClock, we should make sure that no QemuTimers
>>> are still in flight. To implement that, the caller of disabling will
>>> wait until the last user's exit.
>>>
>>> Note, the callers of qemu_clock_enable() should be sync by themselves,
>>> not protected by this patch.
>>>
>>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>>
>> This is an interesting approach.
>>
>>> -    if (!clock->enabled)
>>> -        return;
>>> +    atomic_inc(&clock->using);
>>> +    if (unlikely(!clock->enabled)) {
>>> +        goto exit;
>>> +    }
>>
>> This can return directly, it doesn't need to increment and decrement
>> clock->using.
>>
> Here is a race condition like the following

Ah, I see.

Still this seems a bit backwards.  Most of the time you will have no one
on the wait_using condvar, but you are almost always signaling the
condvar (should be broadcast BTW)...

Paolo