[tip: locking/core] locking/qspinlock: Micro-optimize pending state waiting for unlock

[tip-bot2 for Guo Ren]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     4282494a20cdcaf38d553f2c2ff6f252084f979c
Gitweb:        https://git.kernel.org/tip/4282494a20cdcaf38d553f2c2ff6f252084f979c
Author:        Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
AuthorDate:    Wed, 04 Jan 2023 21:19:52 -05:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 05 Jan 2023 11:01:50 +01:00

locking/qspinlock: Micro-optimize pending state waiting for unlock

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 situation is
the SMT scenarios in the same core. Not an entering low-power state
situation. Of course, the granularity between cores is "cacheline",
but the granularity between SMT hw threads of the same core could
be "byte" which internal LSU handles. For example, when a hw-thread
yields the resources of the core to other hw-threads, this patch
could help the hw-thread stay in the sleep state and prevent it
from being woken up by other hw-threads xchg_tail.

Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230105021952.3090070-1-guoren@xxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 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 2b23378..ebe6b8e 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.