linux-kernel - Re: [PATCH 4/4] locking/rwsem: Rework zeroing reader waiter->task

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Date:	Mon, 9 May 2016 09:50:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	mingo@...nel.org, tglx@...utronix.de, Waiman.Long@....com,
	jason.low2@...com, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 4/4] locking/rwsem: Rework zeroing reader waiter->task

On Sun, May 08, 2016 at 09:56:10PM -0700, Davidlohr Bueso wrote:
> Readers that are awoken will expect a nil ->task indicating
> that a wakeup has occurred. There is a mismatch between the
> smp_mb() and its documentation, in that the serialization is
> done between reading the task and the nil store. Furthermore,
> in addition to having the overlapping of loads and stores to
> waiter->task guaranteed to be ordered within that CPU, both
> wake_up_process() originally and now wake_q_add() already
> imply barriers upon successful calls, which serves the comment.
> 
> Just atomically do a xchg() and simplify the whole thing. We can
> use relaxed semantics as before mentioned in addition to the
> barrier provided by wake_q_add(), delaying there is no risk in
> reordering with the actual wakeup.

> @@ -190,24 +189,18 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
>  	next = sem->wait_list.next;
>  	loop = woken;
>  	do {
> +		struct task_struct *tsk;
> +
>  		waiter = list_entry(next, struct rwsem_waiter, list);
>  		next = waiter->list.next;
> -		tsk = waiter->task;
> -		/*
> -		 * Make sure we do not wakeup the next reader before
> -		 * setting the nil condition to grant the next reader;
> -		 * otherwise we could miss the wakeup on the other
> -		 * side and end up sleeping again. See the pairing
> -		 * in rwsem_down_read_failed().
> -		 */
> -		smp_mb();
> -		waiter->task = NULL;
> +
> +		tsk = xchg_relaxed(&waiter->task, NULL);
>  		wake_q_add(wake_q, tsk);

Not a great fan of this patch; it again doesn't fix the race, and
smp_store_release() is a cheaper option on x86.