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From:   guoren@kernel.org
To:     guoren@kernel.org, David.Laight@ACULAB.COM, will@kernel.org,
        peterz@infradead.org, mingo@redhat.com, longman@redhat.com
Cc:     linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org, Guo Ren <guoren@linux.alibaba.com>
Subject: [PATCH] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked
Date:   Wed, 19 Jul 2023 03:00:01 -0400
Message-Id: <20230719070001.795010-1-guoren@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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

Using arch_spinlock_is_locked would cause another 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().

Signed-off-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Peter Zijlstra <peterz@infradead.org>
---
Changelog:
This patch is separate from:
https://lore.kernel.org/linux-riscv/20220808071318.3335746-1-guoren@kernel.org/

Peter & David have commented on it:
https://lore.kernel.org/linux-riscv/YsK4Z9w0tFtgkni8@hirez.programming.kicks-ass.net/
---
 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