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

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