Re: [Xen-devel] [PATCH v10 03/19] qspinlock: Add pending bit

[Waiman Long <waiman.long@xxxxxx>]

On 05/08/2014 02:57 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:31AM -0400, Waiman Long wrote:
> > +/**
> > + * trylock_pending - try to acquire queue spinlock using the pending bit
> > + * @lock : Pointer to queue spinlock structure
> > + * @pval : Pointer to value of the queue spinlock 32-bit word
> > + * Return: 1 if lock acquired, 0 otherwise
> > + */
> > +static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
> Still don't like you put it in a separate function, but you don't need
> the pointer thing. Note how after you fail the trylock_pending() you
> touch the second (node) cacheline.
>
> > @@ -110,6 +184,9 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
> > val)
> >
> >         BUILD_BUG_ON(CONFIG_NR_CPUS>= (1U<<  _Q_TAIL_CPU_BITS));
> >
> > +       if (trylock_pending(lock,&val))
> > +               return; /* Lock acquired */
> > +
> >         node = this_cpu_ptr(&mcs_nodes[0]);
> >         idx = node->count++;
> >         tail = encode_tail(smp_processor_id(), idx);
> > @@ -119,15 +196,18 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
> > val)
> >         node->next = NULL;
> >
> >         /*
> > +        * we already touched the queueing cacheline; don't bother with pending
> > +        * stuff.
> > +        *
> >          * trylock || xchg(lock, node)
> >          *
> > -        * 0,0 ->  0,1 ; trylock
> > -        * p,x ->  n,x ; prev = xchg(lock, node)
> > +        * 0,0,0 ->  0,0,1 ; trylock
> > +        * p,y,x ->  n,y,x ; prev = xchg(lock, node)
> >          */
> And any value of @val we might have had here is completely out-dated.
> The only thing that makes sense it to set:
>
>         val = 0;
>
> Which makes us start with a trylock, alternatively we can re-read val.
That is true. I will make the change to get rid of the pointer thing.

As for the separate trylock_pending function, my original goal was to 
have a better delineation of different portions of the code.  Given the 
fact that I broke up the slowpath function into 2 in a later patch, I 
may not really need to separate it out. I will pull it back in the next 
version.
-Longman

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