Guo Ren c6f4a90022 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

2023-09-21 10:17:00 +02:00

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