Commit-ID: ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15
Gitweb: https://git.kernel.org/tip/ae75d9089ff7095d1d1a12c3cd86b21d3eaf3b15
Author: Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Thu, 26 Apr 2018 11:34:26 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 27 Apr 2018 09:48:52 +0200
locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() when locking
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@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: boqun.feng@xxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1524738868-31318-13-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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 @@ locked:
* 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);
--
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