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

[Rik van Riel]

On 03/26/2013 10:27 AM, Peter Zijlstra wrote:
> 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?

That makes me wonder, how did mm_take_all_locks used to work before
we turned the anon_vma lock into a mutex?

The code used to use spin_lock_nest_lock, but still has the potential
to overflow the preempt counter. How did that ever work right?

> 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);
> }

That is essentially a read-only version of the global rwlock that
I originally proposed, where the global lock takes the lock for
write and the single version takes the global lock for read, and
then one of the semaphore spinlocks.

I could certainly implement and test the above, unless Linus
thinks it's too ugly to live :)

> 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?

Not without breaking your scheme above :)

I suppose making things into a sleeping lock should be possible,
but that is another major change in this code. I would rather do
things in smaller steps...

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