Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
> >>@@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >> 		spin_unlock_mutex(&lock->wait_lock, flags);
> >> 		schedule_preempt_disabled();
> >>
> >>-		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> >>+		if (use_ww_ctx && ww_ctx) {
> >>+			/*
> >>+			 * Always re-check whether we're in first position. We
> >>+			 * don't want to spin if another task with a lower
> >>+			 * stamp has taken our position.
> >>+			 *
> >>+			 * We also may have to set the handoff flag again, if
> >>+			 * our position at the head was temporarily taken away.
> >>+			 */
> >>+			first = __mutex_waiter_is_first(lock, &waiter);
> >>+
> >>+			if (first)
> >>+				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> >>+		} else if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> >> 			first = true;
> >> 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> >> 		}
> >
> >So the point is that !ww_ctx entries are 'skipped' during the insertion
> >and therefore, if one becomes first, it must stay first?
> 
> Yes. Actually, it should be possible to replace all the cases of use_ww_ctx
> || first with ww_ctx. Similarly, all cases of use_ww_ctx && ww_ctx could be
> replaced by just ww_ctx.


I'm not seeing how "use_ww_ctx || first" -> "ww_ctx" works. And while
"use_ww_ctx && ww_ctx" -> "ww_ctx" is correct, it didn't work right on
some older GCCs, they choked on value propagation for ww_ctx and kept
emitting code even if we passed in NULL. Hence use_ww_ctx.

Arnd is now looking to raise the minimum supported GCC version, so maybe
we should look at that again if he gets anywhere.