On 16.12.2016 18:20, Peter Zijlstra wrote:
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.
My bad, missing the '|| first'.
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.
Okay, I'll stick with use_ww_ctx. Thanks for the explanation.
Nicolai
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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