linux-kernel - [PATCH V2] asm-generic: ticket-lock: Optimize arch_spin_value

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Date:   Sun, 30 Jul 2023 22:33:08 -0400
From:   guoren@...nel.org
To:     guoren@...nel.org, David.Laight@...LAB.COM, will@...nel.org,
        peterz@...radead.org, mingo@...hat.com, longman@...hat.com
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH V2] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked

From: Guo Ren <guoren@...ux.alibaba.com>

The arch_spin_value_unlocked would cause an unnecessary memory
access to the contended value. Although it won't cause a significant
performance gap in most architectures, the arch_spin_value_unlocked
argument contains enough information. Thus, remove unnecessary
atomic_read in arch_spin_value_unlocked().

The caller of arch_spin_value_unlocked() could benefit from this
change. Currently, the only caller is lockref.

Signed-off-by: Guo Ren <guoren@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: David Laight <David.Laight@...LAB.COM>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
---
Changelog
V2:
 - Fixup commit log with Waiman advice.
 - Add Waiman comment in the commit msg.
---
 include/asm-generic/spinlock.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
index fdfebcb050f4..90803a826ba0 100644
--- a/include/asm-generic/spinlock.h
+++ b/include/asm-generic/spinlock.h
@@ -68,11 +68,18 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 	smp_store_release(ptr, (u16)val + 1);
 }
 
+static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
+{
+	u32 val = lock.counter;
+
+	return ((val >> 16) == (val & 0xffff));
+}
+
 static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	u32 val = atomic_read(lock);
+	arch_spinlock_t val = READ_ONCE(*lock);
 
-	return ((val >> 16) != (val & 0xffff));
+	return !arch_spin_value_unlocked(val);
 }
 
 static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
@@ -82,11 +89,6 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
 	return (s16)((val >> 16) - (val & 0xffff)) > 1;
 }
 
-static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
-{
-	return !arch_spin_is_locked(&lock);
-}
-
 #include <asm/qrwlock.h>
 
 #endif /* __ASM_GENERIC_SPINLOCK_H */
-- 
2.36.1