linux-kernel - [PATCH] locking/pvqspinlock/x86: Remove redundant CMP after CMPXCHG in __raw_callee_save___pv_queued_spin

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Message-ID: <20240412083908.282802-1-ubizjak@gmail.com>
Date: Fri, 12 Apr 2024 10:38:53 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: [PATCH] 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@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 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 ef9697f20129..466af57b8ed6 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							\
-- 
2.44.0