On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
@@ -677,15 +722,25 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
debug_mutex_lock_common(lock, &waiter);
debug_mutex_add_waiter(lock, &waiter, task);
- /* add waiting tasks to the end of the waitqueue (FIFO): */
- list_add_tail(&waiter.list, &lock->wait_list);
+ lock_contended(&lock->dep_map, ip);
+
+ if (!use_ww_ctx) {
+ /* add waiting tasks to the end of the waitqueue (FIFO): */
+ list_add_tail(&waiter.list, &lock->wait_list);
+ } else {
+ /* Add in stamp order, waking up waiters that must back off. */
+ ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
+ if (ret)
+ goto err_early_backoff;
+
+ waiter.ww_ctx = ww_ctx;
+ }
+
waiter.task = task;
Would an unconditional waiter.ww_ctx = ww_ctx be chep enough? (Same
cacheline write and all that?)
Makes the above clearer in that you have
if (!ww_ctx) {
list_add_tail();
} else {
ret = __ww_mutex_add_waiter(); /* no need to handle !ww_ctx */
if (ret)
goto err_early_backoff;
}
waiter.ww_ctx = ww_ctx;
waiter.task = task;
if (__mutex_waiter_is_first(lock, &waiter))
__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
- lock_contended(&lock->dep_map, ip);
-
set_task_state(task, state);
for (;;) {
/*
@@ -693,8 +748,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
* mutex_unlock() handing the lock off to us, do a trylock
* before testing the error conditions to make sure we pick up
* the handoff.
+ *
+ * For w/w locks, we always need to do this even if we're not
+ * currently the first waiter, because we may have been the
+ * first waiter during the unlock.
*/
- if (__mutex_trylock(lock, first))
+ if (__mutex_trylock(lock, use_ww_ctx || first))
I'm not certain about the magic of first vs HANDOFF. Afaict, first ==
HANDOFF and this patch breaks that relationship. I think you need to add
bool handoff; as a separate tracker to first.
goto acquired;
/*
@@ -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.
Comment makes sense.
Ah. Should this be just if (use_ww_ctx) { /* always recheck... */ ?
Except that !ww_ctx are never gazzumped in the list, so if they are
first, then they are always first.
Could you explain that as well (about why !ww_ctx is special here but
not above). And then it can even be reduced to if (ww_ctx) {} to match
the first chunk if the revision is acceptable.
-Chris