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Subject[PATCH V2] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked
Date
From: Guo Ren <guoren@linux.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@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Guo Ren <guoren@linux.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
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