Re: [Xen-devel] [PATCH 03/11] qspinlock: Add pending bit

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Jun 17, 2014 at 04:36:15PM -0400, Konrad Rzeszutek Wilk wrote:
> On Sun, Jun 15, 2014 at 02:47:00PM +0200, Peter Zijlstra wrote:
> > Because the qspinlock needs to touch a second cacheline; add a pending
> > bit and allow a single in-word spinner before we punt to the second
> > cacheline.
> 
> Could you add this in the description please:
> 
> And by second cacheline we mean the local 'node'. That is the:
> mcs_nodes[0] and mcs_nodes[idx]

Those should be the very same cacheline :), but yes, I can add something
like that.

> Perhaps it might be better then to split this in the header file
> as this is trying to not be a slowpath code - but rather - a
> pre-slow-path-lets-try-if-we can do another cmpxchg in case
> the unlocker has just unlocked itself.
> 
> So something like:
> 
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index e8a7ae8..29cc9c7 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -75,11 +75,21 @@ extern void queue_spin_lock_slowpath(struct qspinlock 
> *lock, u32 val);
>   */
>  static __always_inline void queue_spin_lock(struct qspinlock *lock)
>  {
> -     u32 val;
> +     u32 val, new;
>  
>       val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
>       if (likely(val == 0))
>               return;
> +
> +     /* One more attempt - but if we fail mark it as pending. */
> +     if (val == _Q_LOCKED_VAL) {
> +             new = Q_LOCKED_VAL |_Q_PENDING_VAL;
> +
> +             old = atomic_cmpxchg(&lock->val, val, new);
> +             if (old == _Q_LOCKED_VAL) /* YEEY! */
> +                     return;
> +             val = old;
> +     }
>       queue_spin_lock_slowpath(lock, val);
>  }

I think that's too big for an inline function.

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