Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2013-03-26 at 06:40 -0700, Michel Lespinasse wrote:

> sem_nsems is user provided as the array size in some semget system
> call. It's the size of an ipc semaphore array.

So we're basically adding a random (big) number to preempt_count
(obviously while preemption is disabled), seems rather costly and
undesirable.

> complex semop operations take the array's lock plus every semaphore
> locks; simple semop operations (operating on a single semaphore) only
> take that one semaphore's lock.

Right, standard global/local lock like stuff. Is there a way we can add
a r/o test to the 'local' lock operation and avoid doing the above?

Maybe something like:

void sma_lock(struct sem_array *sma) /* global */
{
	int i;

	sma->global_locked = 1;
	smp_wmb(); /* can we merge with the LOCK ? */
	spin_lock(&sma->global_lock);

	/* wait for all local locks to go away */
	for (i = 0; i < sma->sem_nsems; i++)
		spin_unlock_wait(&sem->sem_base[i]->lock);	
}

void sma_lock_one(struct sem_array *sma, int nr) /* local */
{
	smp_rmb(); /* pairs with wmb in sma_lock() */
	if (unlikely(sma->global_locked)) { /* wait for global lock */
		while (sma->global_locked)
			spin_unlock_wait(&sma->global_lock);
	}
	spin_lock(&sma->sem_base[nr]->lock);
}

This still has the problem of a non-preemptible section of O(sem_nsems) 
(with the avg wait-time on the local lock). Could we make the global 
lock a sleeping lock?