From ce3576ebd62d99f79c1dc98824e2ef6d6ab68434 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Wed, 24 Jan 2024 11:49:53 +0100 Subject: [PATCH] locking/rtmutex: Use try_cmpxchg_relaxed() in mark_rt_mutex_waiters() Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old. The x86 CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after CMPXCHG (and related move instruction in front of CMPXCHG). Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when CMPXCHG fails. There is no need to re-read the value in the loop. Note that the value from *ptr should be read using READ_ONCE() to prevent the compiler from merging, refetching or reordering the read. No functional change intended. Signed-off-by: Uros Bizjak Signed-off-by: Ingo Molnar Cc: Peter Zijlstra Cc: Waiman Long Cc: Will Deacon Cc: Thomas Gleixner Cc: Linus Torvalds Cc: Paul E. McKenney Link: https://lore.kernel.org/r/20240124104953.612063-1-ubizjak@gmail.com --- kernel/locking/rtmutex.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 4a10e8c16fd2..88d08eeb8bc0 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -237,12 +237,13 @@ static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock, */ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock) { - unsigned long owner, *p = (unsigned long *) &lock->owner; + unsigned long *p = (unsigned long *) &lock->owner; + unsigned long owner, new; + owner = READ_ONCE(*p); do { - owner = *p; - } while (cmpxchg_relaxed(p, owner, - owner | RT_MUTEX_HAS_WAITERS) != owner); + new = owner | RT_MUTEX_HAS_WAITERS; + } while (!try_cmpxchg_relaxed(p, &owner, new)); /* * The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE