[PATCH 1/4] locking/rtmutex: Implement lockless top-waiter wakeup

Davidlohr Bueso
2015-05-19T17:25:29Z

Mark the task for later wakeup after the wait_lock has been released.
This way, once the next task is awoken, it will have a better chance
to of finding the wait_lock free when continuing executing in
__rt_mutex_slowlock() when trying to acquire the rtmutex, calling
try_to_take_rt_mutex(). Upon contended scenarios, other tasks attempting
take the lock may acquire it first, right after the wait_lock is released,
but (a) this can also occur with the current code, as it relies on the
spinlock fairness, and (b) we are dealing with the top-waiter anyway,
so it will always take the lock next.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/locking/rtmutex.c | 21 ++++++++++-----------
1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 36573e9..74188d8 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -955,14 +955,13 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
}

/*
- * Wake up the next waiter on the lock.
- *
* Remove the top waiter from the current tasks pi waiter list and
- * wake it up.
+ * queue it up.
*
* Called with lock->wait_lock held.
*/
-static void wakeup_next_waiter(struct rt_mutex *lock)
+static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
+ struct rt_mutex *lock)
{
struct rt_mutex_waiter *waiter;
unsigned long flags;
@@ -991,12 +990,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock)

raw_spin_unlock_irqrestore(&current->pi_lock, flags);

- /*
- * It's safe to dereference waiter as it cannot go away as
- * long as we hold lock->wait_lock. The waiter task needs to
- * acquire it in order to dequeue the waiter.
- */
- wake_up_process(waiter->task);
+ wake_q_add(wake_q, waiter->task);
}

/*
@@ -1255,6 +1249,8 @@ static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
static void __sched
rt_mutex_slowunlock(struct rt_mutex *lock)
{
+ WAKE_Q(wake_q);
+
raw_spin_lock(&lock->wait_lock);

debug_rt_mutex_unlock(lock);
@@ -1303,10 +1299,13 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
/*
* The wakeup next waiter path does not suffer from the above
* race. See the comments there.
+ *
+ * Queue the next waiter for wakeup once we release the wait_lock.
*/
- wakeup_next_waiter(lock);
+ mark_wakeup_next_waiter(&wake_q, lock);

raw_spin_unlock(&lock->wait_lock);
+ wake_up_q(&wake_q);

/* Undo pi boosting if necessary: */
rt_mutex_adjust_prio(current);
--
2.1.4