[PATCH V3] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked

[guoren]

From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>

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.

Acked-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
---
Changelog
V3:
 - Add Acked-by tags
 - Optimize commit log

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