rtmutex: Simplify rtmutex_slowtrylock()

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>

kernel/locking/rtmutex.c

@@ -960,22 +960,31 @@ 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;
 
-	raw_spin_lock(&lock->wait_lock);
+	/*
+	 * 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;
 
-	if (likely(rt_mutex_owner(lock) != current)) {
+	/*
+	 * The mutex has currently no owner. Lock the wait lock and
+	 * try to acquire the lock.
+	 */
+	raw_spin_lock(&lock->wait_lock);
 
 	ret = try_to_take_rt_mutex(lock, current, NULL);
+
 	/*
-		 * try_to_take_rt_mutex() sets the lock waiters
-		 * bit unconditionally. Clean this up.
+	 * 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);