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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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.

>       for (;;) {
>               new = _Q_LOCKED_VAL;
>               if (val)
> -                     new = tail | (val & _Q_LOCKED_MASK);
> +                     new = tail | (val & _Q_LOCKED_PENDING_MASK);
>  
>               old = atomic_cmpxchg(&lock->val, val, new);
>               if (old == val)

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