The following commit has been merged into the locking/core branch of tip:
Commit-ID: ce3576ebd62d99f79c1dc98824e2ef6d6ab68434
Gitweb: https://git.kernel.org/tip/ce3576ebd62d99f79c1dc98824e2ef6d6ab68434
Author: Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate: Wed, 24 Jan 2024 11:49:53 +01:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 01 Mar 2024 13:02:05 +01:00
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 <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240124104953.612063-1-ubizjak@xxxxxxxxx
---
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 4a10e8c..88d08ee 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
|