Commit ce3576eb authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo 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
parent e807c2a3

kernel/locking/rtmutex.c

+5 −4
Original line number Diff line number Diff line
@@ -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