Re: [Qemu-devel] [RFC v2 3/5] timer: make qemu_clock_enable sync between disable and timer's cb

[Paolo Bonzini <pbonzini@redhat.com>]

Il 30/07/2013 04:42, liu ping fan ha scritto:
> On Mon, Jul 29, 2013 at 7:21 PM, Paolo Bonzini <pbonzini@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 <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)...
>>
> 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