Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 16, 2021 at 11:41:19AM -0700, Andy Lutomirski wrote:
> mmgrab() and mmdrop() would be better if they were not full barriers.  As a
> trivial optimization,

> mmgrab() could use a relaxed atomic and mmdrop()
> could use a release on architectures that have these operations.

mmgrab() *is* relaxed, mmdrop() is a full barrier but could trivially be
made weaker once membarrier stops caring about it.

static inline void mmdrop(struct mm_struct *mm)
{
	unsigned int val = atomic_dec_return_release(&mm->mm_count);
	if (unlikely(!val)) {
		/* Provide REL+ACQ ordering for free() */
		smp_acquire__after_ctrl_dep();
		__mmdrop(mm);
	}
}

It's slightly less optimal for not being able to use the flags from the
decrement. Or convert the whole thing to refcount_t (if appropriate)
which already does something like the above.