[PATCH] locking/rtmutex: Optimize setting task running after being blocked

Davidlohr Bueso
2015-02-02T06:16:32Z

We explicitly mark the task running after returning from
a __rt_mutex_slowlock() call, which does the actual sleeping
via wait-wake-trylocking. As such, this patch does two things:

(1) refactors the code so that setting current to TASK_RUNNING
is done by __rt_mutex_slowlock(), and not by the callers. The
downside to this is that it becomes a bit unclear when at what
point we block. As such I've added a comment that the task
blocks when calling __rt_mutex_slowlock() so readers can figure
out when it is running again.

(2) relaxes setting current's state through __set_current_state(),
instead of it's more expensive barrier alternative. There was no
need for the implied barrier as we're obviously not planning on
blocking.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/locking/rtmutex.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 7c98873..3059bc2f 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1130,6 +1130,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
set_current_state(state);
}

+ __set_current_state(TASK_RUNNING);
return ret;
}

@@ -1188,10 +1189,9 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);

if (likely(!ret))
+ /* sleep on the mutex */
ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);

- set_current_state(TASK_RUNNING);
-
if (unlikely(ret)) {
remove_waiter(lock, &waiter);
rt_mutex_handle_deadlock(ret, chwalk, &waiter);
@@ -1626,10 +1626,9 @@ int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,

set_current_state(TASK_INTERRUPTIBLE);

+ /* sleep on the mutex */
ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);

- set_current_state(TASK_RUNNING);
-
if (unlikely(ret))
remove_waiter(lock, waiter);

--
2.1.4