[PATCH V11 04/17] locking/qspinlock: Improve xchg_tail for number of cpus >= 16k

[guoren at kernel.org]

From: Guo Ren <guoren at linux.alibaba.com>

The target of xchg_tail is to write the tail to the lock value, so
adding prefetchw could help the next cmpxchg step, which may
decrease the cmpxchg retry loops of xchg_tail. Some processors may
utilize this feature to give a forward guarantee, e.g., RISC-V
XuanTie processors would block the snoop channel & irq for several
cycles when prefetch.w instruction (from Zicbop extension) retired,
which guarantees the next cmpxchg succeeds.

Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
Signed-off-by: Guo Ren <guoren at kernel.org>
---
 kernel/locking/qspinlock.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index d3f99060b60f..96b54e2ade86 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -223,7 +223,10 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
  */
 static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
 {
-	u32 old, new, val = atomic_read(&lock->val);
+	u32 old, new, val;
+
+	prefetchw(&lock->val);
+	val = atomic_read(&lock->val);
 
 	for (;;) {
 		new = (val & _Q_LOCKED_PENDING_MASK) | tail;
-- 
2.36.1