linux-kernel - Re: [PATCH 1/3] wait-simple: Introduce the simple waitqueue implementation

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Date:	Thu, 12 Dec 2013 12:20:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] wait-simple: Introduce the simple waitqueue
 implementation

On Thu, Dec 12, 2013 at 12:18:09PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 11, 2013 at 08:06:37PM -0500, Paul Gortmaker wrote:
> > +unsigned int
> > +__swake_up_locked(struct swait_queue_head *head, unsigned int state,
> > +		  unsigned int num)
> > +{
> > +	struct swaiter *curr, *next;
> > +	int woken = 0;
> > +
> > +	list_for_each_entry_safe(curr, next, &head->task_list, node) {
> > +		if (wake_up_state(curr->task, state)) {
> > +			__swait_dequeue(curr);
> > +			/*
> > +			 * The waiting task can free the waiter as
> > +			 * soon as curr->task = NULL is written,
> > +			 * without taking any locks. A memory barrier
> > +			 * is required here to prevent the following
> > +			 * store to curr->task from getting ahead of
> > +			 * the dequeue operation.
> > +			 */
> > +			smp_wmb();
> > +			curr->task = NULL;
> > +			if (++woken == num)
> > +				break;
> > +		}
> > +	}
> > +	return woken;
> > +}
> > +
> > +unsigned int
> > +__swake_up(struct swait_queue_head *head, unsigned int state, unsigned int num)
> > +{
> > +	unsigned long flags;
> > +	int woken;
> > +
> > +	if (!swaitqueue_active(head))
> > +		return 0;
> > +
> > +	raw_spin_lock_irqsave(&head->lock, flags);
> > +	woken = __swake_up_locked(head, state, num);
> > +	raw_spin_unlock_irqrestore(&head->lock, flags);
> > +	return woken;
> > +}
> > +EXPORT_SYMBOL(__swake_up);
> 
> Urgh, fail. Do not put unbounded loops in raw_spin_lock.
> 
> I think I posted a patch a while back to cure this.

tada!

lkml.kernel.org/r/20131004145625.GN3081@...ns.programming.kicks-ass.net
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