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 <o...@redhat.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
kernel/locking/rtmutex.c | 32 +++++++++++++++++++++-----------
1 file changed, 21 insertions(+), 11 deletions(-)
Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -963,22 +963,32 @@ rt_mutex_slowlock(struct rt_mutex *lock,
/*
* 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;
+ /*
+ * trylock is taking a chance. So we dont have to take
+ * @lock->wait_lock to figure out whether @lock has a real or
+ * if @lock owner is NULL and the RT_MUTEX_HAS_WAITERS bit is
+ * set.
+ */
+ if (rt_mutex_owner(lock))
+ return 0;
+
+ /*
+ * The mutex has currently no owner. Lock the wait lock and
+ * try to acquire the lock.
+ */
raw_spin_lock(&lock->wait_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);
--
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