From ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15 Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@arm.com>
Date: Thu, 26 Apr 2018 11:34:26 +0100
Subject: [PATCH 12/12] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking
Git-commit: ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15
Patch-mainline: v4.18-rc1
References: bsc#1050549
When reaching the head of an uncontended queue on the qspinlock slow-path,
using a try_cmpxchg() instead of a cmpxchg() operation to transition the
lock work to _Q_LOCKED_VAL generates slightly better code for x86 and
pretty much identical code for arm64.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-13-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/locking/qspinlock.c | 19 +++++++++----------
1 file changed, 9 insertions(+), 10 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 956a12983bd0..46813185957b 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -467,16 +467,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* Otherwise, we only need to grab the lock.
*/
- /* In the PV case we might already have _Q_LOCKED_VAL set */
- if ((val & _Q_TAIL_MASK) == tail) {
- /*
- * The atomic_cond_read_acquire() call above has provided the
- * necessary acquire semantics required for locking.
- */
- old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
- if (old == val)
- goto release; /* No contention */
- }
+ /*
+ * In the PV case we might already have _Q_LOCKED_VAL set.
+ *
+ * The atomic_cond_read_acquire() call above has provided the
+ * necessary acquire semantics required for locking.
+ */
+ if (((val & _Q_TAIL_MASK) == tail) &&
+ atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+ goto release; /* No contention */
/* Either somebody is queued behind us or _Q_PENDING_VAL is set */
set_locked(lock);
--
2.16.4