linux-kernel - Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic

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Date:	Tue, 16 Feb 2016 20:40:51 -0500
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Jason Low <jason.low2@...com>, Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

On 02/16/2016 03:53 AM, Peter Zijlstra wrote:
> On Mon, Feb 15, 2016 at 06:22:14PM -0800, Jason Low wrote:
>> On Mon, 2016-02-15 at 18:15 -0800, Jason Low wrote:
>>> On Fri, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote:
>>>> On Fri, 12 Feb 2016, Peter Zijlstra wrote:
>>>>
>>>>> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
>>>>>>   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() ?
>>>> afaict because mutex_can_spin_on_owner() fails immediately when the counter
>>>> is -1, which is a nono for the waiters case.
>>> mutex_can_spin_on_owner() returns false if the task needs to reschedule
>>> or if the lock owner is not on_cpu. In either case, the task will end up
>>> not spinning when it enters the spin loop. So it makes sense if the
>>> waiter also checks mutex_can_spin_on_owner() so that the optimistic spin
>>> queue overhead can be avoided in those cases.
>> Actually, since waiters bypass the optimistic spin queue, that means the
>> the mutex_can_spin_on_owner() isn't really beneficial. So Waiman is
>> right in that it's fine to skip this in the waiter case.
> My concern was the 'pointless' divergence between the code-paths. The
> less they diverge the easier it is to understand and review.
>
> If it doesn't hurt, please keep it the same. If it does need to diverge,
> include a comment on why.

I will keep the preemption, but will still leave out the 
mutex_can_spin_on_owner() check for waiter. I will add a comment to 
document that.

Cheers,
Longman