[PATCH RT 03/36] rtmutex: Simplify rtmutex_slowtrylock()

Steven Rostedt
2015-03-12T19:22:12Z

[-- Attachment #1: 0003-rtmutex-Simplify-rtmutex_slowtrylock.patch --]
[-- Type: text/plain, Size: 2456 bytes --]

3.12.38-rt53-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@linutronix.de>

upstream-commit: 88f2b4c15e561bb5c28709d666364f273bf54b98

Oleg noticed that rtmutex_slowtrylock() has a pointless check for
rt_mutex_owner(lock) != current.

To avoid calling try_to_take_rtmutex() we really want to check whether
the lock has an owner at all or whether the trylock failed because the
owner is NULL, but the RT_MUTEX_HAS_WAITERS bit is set. This covers
the lock is owned by caller situation as well.

We can actually do this check lockless. trylock is taking a chance
whether we take lock->wait_lock to do the check or not.

Add comments to the function while at it.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Conflicts:
kernel/rtmutex.c
---
kernel/rtmutex.c | 33 +++++++++++++++++++++------------
1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 41c42591ef8c..85e03e15c7f9 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -1392,24 +1392,33 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
/*
* Slow path try-lock function:
*/
-static inline int
-rt_mutex_slowtrylock(struct rt_mutex *lock)
+static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
{
- int ret = 0;
+ int ret;

+ /*
+ * If the lock already has an owner we fail to get the lock.
+ * This can be done without taking the @lock->wait_lock as
+ * it is only being read, and this is a trylock anyway.
+ */
+ if (rt_mutex_owner(lock))
+ return 0;
+
+ /*
+ * The mutex has currently no owner. Lock the wait lock and
+ * try to acquire the lock.
+ */
if (!raw_spin_trylock(&lock->wait_lock))
- return ret;
+ return 0;
init_lists(lock);

- if (likely(rt_mutex_owner(lock) != current)) {
+ ret = try_to_take_rt_mutex(lock, current, NULL);

- ret = try_to_take_rt_mutex(lock, current, NULL);
- /*
- * try_to_take_rt_mutex() sets the lock waiters
- * bit unconditionally. Clean this up.
- */
- fixup_rt_mutex_waiters(lock);
- }
+ /*
+ * try_to_take_rt_mutex() sets the lock waiters bit
+ * unconditionally. Clean this up.
+ */
+ fixup_rt_mutex_waiters(lock);

raw_spin_unlock(&lock->wait_lock);

--
2.1.4