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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote:
> 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 val)
> >>    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.

But you did that read _before_ you touched a cold cacheline, that's 100s
of cycles. Whatever value you read back then is now complete nonsense.

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