From c6f4a90022524d06f6d9de323b1757031dcf0c26 Mon Sep 17 00:00:00 2001 From: Guo Ren Date: Fri, 8 Sep 2023 11:43:39 -0400 Subject: asm-generic: ticket-lock: Optimize arch_spin_value_unlocked() The arch_spin_value_unlocked() of ticket-lock would cause the compiler to generate inefficient asm code in riscv architecture because of unnecessary memory access to the contended value. Before the patch: void lockref_get(struct lockref *lockref) { 78: fd010113 add sp,sp,-48 7c: 02813023 sd s0,32(sp) 80: 02113423 sd ra,40(sp) 84: 03010413 add s0,sp,48 0000000000000088 <.LBB296>: CMPXCHG_LOOP( 88: 00053783 ld a5,0(a0) After the patch: void lockref_get(struct lockref *lockref) { CMPXCHG_LOOP( 78: 00053783 ld a5,0(a0) After the patch, the lockref_get() could get in a fast path instead of the function's prologue. This is because ticket lock complex logic would limit compiler optimization for the spinlock fast path, and qspinlock won't. The caller of arch_spin_value_unlocked() could benefit from this change. Currently, the only caller is lockref. Signed-off-by: Guo Ren Signed-off-by: Guo Ren Signed-off-by: Ingo Molnar Acked-by: Waiman Long Acked-by: Will Deacon Link: https://lore.kernel.org/r/20230908154339.3250567-1-guoren@kernel.org --- include/asm-generic/spinlock.h | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) (limited to 'include/asm-generic') 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 #endif /* __ASM_GENERIC_SPINLOCK_H */ -- cgit v1.2.3