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Date: Mon, 8 Aug 2016 19:29:38 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Jason Low <jason.low2@hpe.com>, Davidlohr Bueso <dave@stgolabs.net>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, Imre Deak <imre.deak@intel.com>
Subject: Re: [PATCH v4 2/3] locking/mutex: Enable optimistic spinning of
 woken task in wait queue
Message-ID: <20160808172938.GY6862@twins.programming.kicks-ass.net>
References: <1468874366-56955-1-git-send-email-Waiman.Long@hpe.com>
 <1468874366-56955-3-git-send-email-Waiman.Long@hpe.com>
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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);