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

[Waiman Long <waiman.long@hpe.com>]

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