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Subject[PATCH] locking/qspinlock: Optimize pending state waiting for unlock
Date
From: Guo Ren <guoren@linux.alibaba.com>

When we're pending, we only care about lock value. The xchg_tail
wouldn't affect the pending state. That means the hardware thread
could stay in a sleep state and leaves the rest execution units'
resources of pipeline to other hardware threads. This optimization
may work only for SMT scenarios because the granularity between
cores is cache-block.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
---
kernel/locking/qspinlock.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 2b23378775fe..ebe6b8ec7cb3 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -371,7 +371,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
/*
* We're pending, wait for the owner to go away.
*
- * 0,1,1 -> 0,1,0
+ * 0,1,1 -> *,1,0
*
* this wait loop must be a load-acquire such that we match the
* store-release that clears the locked bit and create lock
@@ -380,7 +380,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* barriers.
*/
if (val & _Q_LOCKED_MASK)
- atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
+ smp_cond_load_acquire(&lock->locked, !VAL);

/*
* take ownership and clear the pending bit.
--
2.36.1
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