Il 30/07/2013 04:42, liu ping fan ha scritto:
> On Mon, Jul 29, 2013 at 7:21 PM, Paolo Bonzini <pbonz...@redhat.com> wrote:
>> Il 29/07/2013 10:10, liu ping fan ha scritto:
>>> On Mon, Jul 29, 2013 at 2:30 PM, Paolo Bonzini <pbonz...@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 <pingf...@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)...
>>
> I have tried to filter out the normal case by
> + qemu_mutex_lock(&clock->lock);
> + if (atomic_fetch_dec(&clock->using) == 1) { ---------> 1st place
> + if (unlikely(!clock->enabled)) { -------> 2nd place
> + qemu_cond_signal(&clock->wait_using);
> + }
> + }
> + qemu_mutex_unlock(&clock->lock);
>
> Could I do it better?
Hmm, do we even need clock->using at this point? For example:
qemu_clock_enable()
{
clock->enabled = enabled;
...
if (!enabled) {
/* If another thread is within qemu_run_timers,
* wait for it to finish.
*/
qemu_event_wait(&clock->callbacks_done_event);
}
}
qemu_run_timers()
{
qemu_event_reset(&clock->callbacks_done_event);
if (!clock->enabled) {
goto out;
}
...
out:
qemu_event_set(&eclock->callbacks_done_event);
}
In the fast path this only does two atomic operations (an OR for reset,
and XCHG for set).
Paolo
