[tip: locking/core] locking/pvqspinlock/x86: Remove redundant CMP after CMPXCHG in __raw_callee_save___pv_queued_spin_unlock()

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     91095666125a666c8f20c2323b742c53165c0325
Gitweb:        https://git.kernel.org/tip/91095666125a666c8f20c2323b742c53165c0325
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Fri, 12 Apr 2024 10:38:53 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 12 Apr 2024 11:42:39 +02:00

locking/pvqspinlock/x86: Remove redundant CMP after CMPXCHG in __raw_callee_save___pv_queued_spin_unlock()

x86 CMPXCHG instruction returns success in the ZF flag. Remove
redundant CMP instruction after CMPXCHG that performs the same check.

Also update the function comment to mention the modern version
of the equivalent C code.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240412083908.282802-1-ubizjak@xxxxxxxxx
---
 arch/x86/include/asm/qspinlock_paravirt.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/qspinlock_paravirt.h b/arch/x86/include/asm/qspinlock_paravirt.h
index ef9697f..466af57 100644
--- a/arch/x86/include/asm/qspinlock_paravirt.h
+++ b/arch/x86/include/asm/qspinlock_paravirt.h
@@ -25,9 +25,9 @@ __PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath, ".spinlock.text");
  *
  * void __lockfunc __pv_queued_spin_unlock(struct qspinlock *lock)
  * {
- *	u8 lockval = cmpxchg(&lock->locked, _Q_LOCKED_VAL, 0);
+ *	u8 lockval = _Q_LOCKED_VAL;
  *
- *	if (likely(lockval == _Q_LOCKED_VAL))
+ *	if (try_cmpxchg(&lock->locked, &lockval, 0))
  *		return;
  *	pv_queued_spin_unlock_slowpath(lock, lockval);
  * }
@@ -43,7 +43,6 @@ __PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath, ".spinlock.text");
 	"mov   $0x1,%eax\n\t"						\
 	"xor   %edx,%edx\n\t"						\
 	LOCK_PREFIX "cmpxchg %dl,(%rdi)\n\t"				\
-	"cmp   $0x1,%al\n\t"						\
 	"jne   .slowpath\n\t"						\
 	"pop   %rdx\n\t"						\
 	FRAME_END							\