Re: [Xen-devel] [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> If unfair lock is supported, the lock acquisition loop at the end of
> the queue_spin_lock_slowpath() function may need to detect the fact
> the lock can be stolen. Code are added for the stolen lock detection.
> 
> A new qhead macro is also defined as a shorthand for mcs.locked.

NAK, unfair should be a pure test-and-set lock.

>  /**
>   * get_qlock - Set the lock bit and own the lock
> - * @lock: Pointer to queue spinlock structure
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 otherwise
>   *
>   * This routine should only be called when the caller is the only one
>   * entitled to acquire the lock.
>   */
> -static __always_inline void get_qlock(struct qspinlock *lock)
> +static __always_inline int get_qlock(struct qspinlock *lock)
>  {
>       struct __qspinlock *l = (void *)lock;
>  
>       barrier();
>       ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
>       barrier();
> +     return 1;
>  }

and here you make a horribly named function more horrible;
try_set_locked() is that its now.

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