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