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Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

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On 02/12/2016 03:23 PM, Peter Zijlstra wrote:
> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
>> This patch adds a new waiter parameter to the mutex_optimistic_spin()
>> function to prepare it to be used by a waiter-spinner that doesn't
>> need to go into the OSQ as there can only be one waiter-spinner which
>> is the head of the waiting queue.
> Does not explain why..
>
>>   static bool mutex_optimistic_spin(struct mutex *lock,
>> +				  struct ww_acquire_ctx *ww_ctx,
>> +				  const bool use_ww_ctx, int waiter)
>>   {
>>   	struct task_struct *task = current;
>> +	bool acquired = false;
>>
>> +	if (!waiter) {
>> +		if (!mutex_can_spin_on_owner(lock))
>> +			goto done;
> Why doesn't the waiter have to check mutex_can_spin_on_owner() ?

The reason why regular spinner need to call mutex_can_spin_on_owner is 
because we want to avoid the overhead of the OSQ if  we can't spin. As 
mutex_can_spin_on_owner() is similar to mutex_spin_on_owner(), the 
waiter-spinner will exit spinning after calling mutex_spin_on_owner().

Cheers,
Longman

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[PATCH v2 0/4] locking/mutex: Enable optimistic spinning of lock waiter Waiman Long <Waiman.Long@hpe.com> - 2016-02-12 18:40 +0100
  [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <Waiman.Long@hpe.com> - 2016-02-12 18:40 +0100
    Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Peter Zijlstra <peterz@infradead.org> - 2016-02-12 21:30 +0100
      Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Davidlohr Bueso <dave@stgolabs.net> - 2016-02-12 23:20 +0100
        Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Peter Zijlstra <peterz@infradead.org> - 2016-02-13 13:20 +0100
          Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Davidlohr Bueso <dave@stgolabs.net> - 2016-02-13 19:20 +0100
        Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Jason Low <jason.low2@hp.com> - 2016-02-16 03:20 +0100
          Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Jason Low <jason.low2@hp.com> - 2016-02-16 03:30 +0100
            Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Peter Zijlstra <peterz@infradead.org> - 2016-02-16 10:00 +0100
              Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <waiman.long@hpe.com> - 2016-02-17 03:00 +0100
      Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <waiman.long@hpe.com> - 2016-02-15 23:10 +0100
    Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Peter Zijlstra <peterz@infradead.org> - 2016-02-12 21:50 +0100
      Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <waiman.long@hpe.com> - 2016-02-16 02:30 +0100
        Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Jason Low <jason.low2@hp.com> - 2016-02-16 04:10 +0100
          Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <waiman.long@hpe.com> - 2016-02-16 04:40 +0100
    Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Davidlohr Bueso <dave@stgolabs.net> - 2016-02-12 23:10 +0100
    Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to  mutex_optimistic_spin() Davidlohr Bueso <dave@stgolabs.net> - 2016-02-12 23:10 +0100
      Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long <waiman.long@hpe.com> - 2016-02-16 01:10 +0100
  [PATCH v2 3/4] locking/mutex: Avoid missed wakeup of mutex waiter Waiman Long <Waiman.Long@hpe.com> - 2016-02-12 18:40 +0100
  [PATCH v2 2/4] locking/mutex: Enable optimistic spinning of woken task in wait queue Waiman Long <Waiman.Long@hpe.com> - 2016-02-12 18:40 +0100
  Re: [PATCH v2 0/4] locking/mutex: Enable optimistic spinning of lock  waiter Peter Zijlstra <peterz@infradead.org> - 2016-02-16 10:00 +0100
    Re: [PATCH v2 0/4] locking/mutex: Enable optimistic spinning of lock  waiter Waiman Long <waiman.long@hpe.com> - 2016-02-17 03:00 +0100

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