Re: [Qemu-devel] [PATCH 1/4] fifolock: create rfifolock_is_locked helper

["Denis V. Lunev" <den@openvz.org>]

On 11/02/2015 04:12 PM, Stefan Hajnoczi wrote:
> On Sun, Nov 01, 2015 at 04:55:51PM +0300, Denis V. Lunev wrote:
>> On 10/30/2015 06:41 PM, Stefan Hajnoczi wrote:
>>> On Wed, Oct 28, 2015 at 06:01:02PM +0300, Denis V. Lunev wrote:
>>>> +int  rfifolock_is_locked(RFifoLock *r);
>>> Please use bool instead of int.
>>>
>>>> diff --git a/util/rfifolock.c b/util/rfifolock.c
>>>> index afbf748..8ac58cb 100644
>>>> --- a/util/rfifolock.c
>>>> +++ b/util/rfifolock.c
>>>> @@ -48,7 +48,7 @@ void rfifolock_lock(RFifoLock *r)
>>>>       /* Take a ticket */
>>>>       unsigned int ticket = r->tail++;
>>>> -    if (r->nesting > 0 && qemu_thread_is_self(&r->owner_thread)) {
>>>> +    if (r->nesting > 0 && rfifolock_is_locked(r)) {
>>>>           r->tail--; /* put ticket back, we're nesting */
>>>>       } else {
>>>>           while (ticket != r->head) {
>>>> @@ -69,10 +69,15 @@ void rfifolock_unlock(RFifoLock *r)
>>>>   {
>>>>       qemu_mutex_lock(&r->lock);
>>>>       assert(r->nesting > 0);
>>>> -    assert(qemu_thread_is_self(&r->owner_thread));
>>>> +    assert(rfifolock_is_locked(r));
>>>>       if (--r->nesting == 0) {
>>>>           r->head++;
>>>>           qemu_cond_broadcast(&r->cond);
>>>>       }
>>>>       qemu_mutex_unlock(&r->lock);
>>>>   }
>>>> +
>>>> +int rfifolock_is_locked(RFifoLock *r)
>>>> +{
>>>> +    return qemu_thread_is_self(&r->owner_thread);
>>>> +}
>>> The function name confused me since "does the current thread hold the
>>> lock?" != "does anyone currently hold the lock?".
>>>
>>> I suggest:
>>>
>>> bool rfifolock_held_by_current_thread(RFifoLock *r) {
>>>      return r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
>>> }
>>>
>>> Then the r->nesting > 0 testing can also be dropped by callers, which is
>>> good since rfifolock_is_locked() does not return a meaningful result
>>> when r->nesting == 0.
>> actually the function is broken in the current state:
>>      aio_context_acquire()
>>      aio_context_release()
>>      aio_context_is_locked() will return true.
>> the problem is that owner thread is not reset on lock release.
> The owner thread field is only valid when nesting > 0.
>
>> with your proposal the function become racy if called from outer
>> thread. I need to think a bit here.
> This is thread-safe:
>
> bool owner;
>
> qemu_mutex_lock(&r->lock);
> owner = r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
> qemu_mutex_unlock(&r->lock);
>
> return owner;
yep, I know.

But I do not want to take the lock for check.
IMHO it would be better to

@@ -68,11 +68,16 @@ void rfifolock_lock(RFifoLock *r)
  void rfifolock_unlock(RFifoLock *r)
  {
      qemu_mutex_lock(&r->lock);
-    assert(r->nesting > 0);
-    assert(qemu_thread_is_self(&r->owner_thread));
+    assert(rfifolock_is_owner(r));
      if (--r->nesting == 0) {
+        qemu_thread_clear(&r->owner_thread);
          r->head++;
          qemu_cond_broadcast(&r->cond);
      }
      qemu_mutex_unlock(&r->lock);
  }
+
+bool rfifolock_is_owner(RFifoLock *r)
+{
+    return r->nesting > 0 && qemu_thread_is_self(&r->owner_thread);
+}

which does not require lock and thread safe.

Den