The following commit has been merged into the locking/core branch of tip:
Commit-ID: c6f4a90022524d06f6d9de323b1757031dcf0c26
Gitweb: https://git.kernel.org/tip/c6f4a90022524d06f6d9de323b1757031dcf0c26
Author: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
AuthorDate: Fri, 08 Sep 2023 11:43:39 -04:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 21 Sep 2023 10:17:00 +02:00
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@xxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230908154339.3250567-1-guoren@xxxxxxxxxx
---
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 fdfebcb..90803a8 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 */
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