Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 20, 2015 at 07:28:16PM +0100, Manfred Spraul wrote:

> >We need the full barrier to serialize STORE's as well, but probably we can
> >rely on control dependancy and thus we only need rmb().
> Do we need a full barrier or not?
> 
> I don't manage to create a proper line of reasoning.

I think I agree with Oleg in that we only need the smp_rmb(); of course
that wants a somewhat elaborate comment to go along with it. How about
something like so:

	spin_unlock_wait(&local);
	/*
	 * The above spin_unlock_wait() forms a control dependency with
	 * any following stores; because we must first observe the lock
	 * unlocked and we cannot speculate stores.
	 *
	 * Subsequent loads however can easily pass through the loads
	 * represented by spin_unlock_wait() and therefore we need the
	 * read barrier.
	 *
	 * This together is stronger than ACQUIRE for @local and
	 * therefore we will observe the complete prior critical section
	 * of @local.
	 */
	 smp_rmb();

The obvious alternative is using spin_unlock_wait() with an
smp_load_acquire(), but that might be more expensive on some archs due
to repeated issuing of memory barriers.