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@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>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 956a129..4681318 100644 (file)
--- 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);