Re: [Xen-devel] [PATCH v11 14/16] pvqspinlock: Add qspinlock para-virtualization support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, May 30, 2014 at 11:44:00AM -0400, Waiman Long wrote:
> @@ -19,13 +19,46 @@ extern struct static_key virt_unfairlocks_enabled;
>   * that the clearing the lock bit is done ASAP without artificial delay
>   * due to compiler optimization.
>   */
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +static __always_inline void __queue_spin_unlock(struct qspinlock *lock)
> +#else
>  static inline void queue_spin_unlock(struct qspinlock *lock)
> +#endif
>  {
>       barrier();
>       ACCESS_ONCE(*(u8 *)lock) = 0;
>       barrier();
>  }
>  
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +/*
> + * The lock byte can have a value of _Q_LOCKED_SLOWPATH to indicate
> + * that it needs to go through the slowpath to do the unlocking.
> + */
> +#define _Q_LOCKED_SLOWPATH   (_Q_LOCKED_VAL | 2)
> +
> +extern void queue_spin_unlock_slowpath(struct qspinlock *lock);
> +
> +static inline void queue_spin_unlock(struct qspinlock *lock)
> +{
> +     barrier();
> +     if (static_key_false(&paravirt_spinlocks_enabled)) {
> +             /*
> +              * Need to atomically clear the lock byte to avoid racing with
> +              * queue head waiter trying to set _QLOCK_LOCKED_SLOWPATH.
> +              */
> +             if (likely(cmpxchg((u8 *)lock, _Q_LOCKED_VAL, 0)
> +                             == _Q_LOCKED_VAL))
> +                     return;
> +             else
> +                     queue_spin_unlock_slowpath(lock);
> +
> +     } else {
> +             __queue_spin_unlock(lock);
> +     }
> +     barrier();
> +}
> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */

Ideally we'd make all this use alternatives or so, such that the actual
function remains short enough to actually inline;

static inline void queue_spin_unlock(struct qspinlock *lock)
{
        pv_spinlock_alternative(
                ACCESS_ONCE(*(u8 *)lock) = 0,
                pv_queue_spin_unlock(lock));
}

Or however that trickery works.

Attachment: pgpT0hGKKqF_T.pgp
Description: PGP signature

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel