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

[Waiman Long]

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.

-Longman
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