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

[Peter Zijlstra]

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.

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