Re: [Xen-devel] [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit path

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, May 30, 2014 at 11:43:52AM -0400, Waiman Long wrote:
> ---
>  kernel/locking/qspinlock.c |   18 ++++++++++++++++--
>  1 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index fc7fd8c..7f10758 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -233,11 +233,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, 
> u32 val)
>        */
>       for (;;) {
>               /*
> -              * If we observe any contention; queue.
> +              * If we observe that the queue is not empty or both
> +              * the pending and lock bits are set, queue
>                */
> -             if (val & ~_Q_LOCKED_MASK)
> +             if ((val & _Q_TAIL_MASK) ||
> +                 (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL)))
>                       goto queue;
>  
> +             if (val == _Q_PENDING_VAL) {
> +                     /*
> +                      * Pending bit is set, but not the lock bit.
> +                      * Assuming that the pending bit holder is going to
> +                      * set the lock bit and clear the pending bit soon,
> +                      * it is better to wait than to exit at this point.
> +                      */
> +                     cpu_relax();
> +                     val = atomic_read(&lock->val);
> +                     continue;
> +             }
> +
>               new = _Q_LOCKED_VAL;
>               if (val == new)
>                       new |= _Q_PENDING_VAL;


So, again, you just posted a new version without replying to the
previous discussion; so let me try again, what's wrong with the proposal
here:

  lkml.kernel.org/r/20140417163640.GT11096@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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