[Xen-devel] [PATCH 06/11] qspinlock: Optimize pending bit

[Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>]

XXX: merge into the pending bit patch..

It is possible so observe the pending bit without the locked bit when
the last owner has just released but the pending owner has not yet
taken ownership.

In this case we would normally queue -- because the pending bit is
already taken. However, in this case the pending bit is guaranteed to
be released 'soon', therefore wait for it and avoid queueing.

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/locking/qspinlock.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux-2.6/kernel/locking/qspinlock.c
===================================================================
--- linux-2.6.orig/kernel/locking/qspinlock.c
+++ linux-2.6/kernel/locking/qspinlock.c
@@ -226,6 +226,16 @@ void queue_spin_lock_slowpath(struct qsp
        BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
 
        /*
+        * wait for in-progress pending->locked hand-overs
+        *
+        * 0,1,0 -> 0,0,1
+        */
+       if (val == _Q_PENDING_VAL) {
+               while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
+                       cpu_relax();
+       }
+
+       /*
         * trylock || pending
         *
         * 0,0,0 -> 0,0,1 ; trylock



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