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Re: [Xen-devel] [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

On 04/17/2014 11:49 AM, Peter Zijlstra wrote:
On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote:
@@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
        node->next = NULL;

+        * We touched a (possibly) cold cacheline; attempt the trylock once
+        * more in the hope someone let go while we weren't watching as long
+        * as no one was queuing.
+       if (!(val&  _Q_TAIL_MASK)&&  queue_spin_trylock(lock))
+               goto release;
But you just did a potentially very expensive op; @val isn't
representative anymore!

That is not true. I pass in a pointer to val to trylock_pending() (the pointer thing) so that it will store the latest value that it reads from the lock back into val. I did miss one in the PV qspinlock exit loop. I will add it back when I do the next version.


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