Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

[Davidlohr Bueso <dave@xxxxxxxxxxxx>]

On Mon, 11 Nov 2024, Suren Baghdasaryan wrote:

> @@ -787,7 +893,10 @@ static inline void vma_start_write(struct vm_area_struct *vma)
> 	 * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy.
> 	 */
> 	WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
> -	up_write(&vma->vm_lock.lock);
> +	/* Write barrier to ensure vm_lock_seq change is visible before count */
> +	smp_wmb();
> +	rwsem_release(&vma->vm_lock.dep_map, _RET_IP_);
> +	atomic_set(&vma->vm_lock.count, VMA_LOCK_UNLOCKED);

Too many barriers here. Just do atomic_set_release and remove that
smp_wmb(). And what you are doing is really ensuring nothing leaks out
of the critical region, so that comment should also be more generic.

I would expect regression testing to catch this sort of thing.

...

> #ifdef CONFIG_PER_VMA_LOCK
> +		struct wait_queue_head vma_writer_wait;

You might want to use rcuwait here instead, which is much more
optimized for the single waiter requirement vmas have.

Thanks,
Davidlohr