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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 <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230908154339.3250567-1-guoren@kernel.org
This commit is contained in:
Guo Ren 2023-09-08 11:43:39 -04:00 committed by Ingo Molnar
parent fbeb558b0d
commit c6f4a90022
1 changed files with 9 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
include/asm-generic/spinlock.h
View File

@ -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 */