linux-kernel - Re: [PATCH v4 2/3] locking/mutex: Enable optimistic spinning of woken task in wait queue

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Date:	Mon, 8 Aug 2016 19:29:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	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>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Imre Deak <imre.deak@...el.com>
Subject: Re: [PATCH v4 2/3] locking/mutex: Enable optimistic spinning of
 woken task in wait queue

On Mon, Jul 18, 2016 at 04:39:25PM -0400, Waiman Long wrote:
> Ding Tianhong reported a live-lock situation where a constant stream
> of incoming optimistic spinners blocked a task in the wait list from
> getting the mutex.
> 
> This patch attempts to fix this live-lock condition by enabling the
> woken task in the wait queue to enter into an optimistic spinning
> loop itself in parallel with the regular spinners in the OSQ. This
> should prevent the live-lock condition from happening.

No, two spinners are not in fact starvation proof. It makes the reported
life-lock scenario much less likely, but it does not guarantee anything.

> +		/*
> +		 * Optimistically spinning on the mutex without the wait lock

There should either be a '.' at the end of that line, or the next line
should not start with a capital.

Also, I don't see how the two sentences are related, should they be in
the same paragraph?

> +		 * The state has to be set to running to avoid another waker
> +		 * spinning on the on_cpu flag while the woken waiter is
> +		 * spinning on the mutex.
> +		 */
> +		acquired = mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx,
> +						 true);
>  		spin_lock_mutex(&lock->wait_lock, flags);
>  	}
>  	__set_task_state(task, TASK_RUNNING);