Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 2006-01-26 at 13:56 -0800, Howard Chu wrote:
> Nick Piggin wrote:

> >>
> >> But why does A take the mutex in the first place? Presumably because 
> >> it is about to execute a critical section. And also presumably, A 
> >> will not release the mutex until it no longer has anything critical 
> >> to do; certainly it could hold it longer if it needed to.
> >>
> >> If A still needed the mutex, why release it and reacquire it, why not 
> >> just hold onto it? The fact that it is being released is significant.
> >>
> >
> > Regardless of why, that is just the simplest scenario I could think
> > of that would give us a test case. However...
> >
> > Why not hold onto it? We sometimes do this in the kernel if we need
> > to take a lock that is incompatible with the lock already being held,
> > or if we discover we need to take a mutex which nests outside our
> > currently held lock in other paths. Ie to prevent deadlock.
> 
> In those cases, A cannot retake the mutex anyway. I.e., you just said 
> that you released the first mutex because you want to acquire a 
> different one. So those cases don't fit this example very well.

Lets say you have two locks X and Y.  Y nests inside of X. To do block1
you need to have lock Y and to do block2 you need to have both locks X
and Y, and block 1 must be done first without holding lock X.

func()
{
again:
	mutex_lock(Y);
	block1();
	if (!mutex_try_lock(X)) {
		mutex_unlock(Y);
		mutex_lock(X);
		mutex_lock(Y);
		if (block1_has_changed()) {
			mutex_unlock(Y);
			mutex_unlock(X);
			goto again;
		}
	}
	block2();
	mutex_unlock(X);
	mutex_unlock(Y);
}

Stuff like the above actually is done (it's done in the kernel). So you
can see here that Y can be released and reacquired right away.  If
another task was waiting on Y (of lower priority) we don't want to give
up the lock, since we would then block and the chances of
block1_has_changed goes up even more.

> 
> > Another reason might be because we will be running for a very long
> > time without requiring the lock.
> 
> And again in this case, A should not be immediately reacquiring the lock 
> if it doesn't actually need it.

I'm not sure what Nick means here, but I'm sure he didn't mean it to
come out that way ;)

> 
> > Or we might like to release it because
> > we expect a higher priority process to take it.
> 
> And in this case, the expected behavior is the same as I've been pursuing.

But you can't know if a higher or lower priority process is waiting.
Sure it works like what you say when a higher priority process is
waiting, but it doesn't when it's a lower priority process waiting.

-- Steve