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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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240124104953.612063-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2024-01-24 11:49:53 +01:00 committed by Ingo Molnar
parent e807c2a370
commit ce3576ebd6
1 changed files with 5 additions and 4 deletions
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9
kernel/locking/rtmutex.c
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@ -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