linux-kernel - Re: [PATCH 3/7] wait.[ch]: Introduce the simple waitqueue (swait) implementation

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Message-ID: <20150114091849.501a0a72@gandalf.local.home>
Date:	Wed, 14 Jan 2015 09:18:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/7] wait.[ch]: Introduce the simple waitqueue (swait)
 implementation

On Wed, 14 Jan 2015 11:38:34 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> 
> 
> So I had a look at this yesterday and came up with the below --
> completely untested etc.
> 
> Now in order to compile test I meant to convert the completion code and
> ran head first into complete_all; it uses spin_lock_irqsave() which
> means it can be used from IRQ context. Now if you look at
> __swake_up_all() you'll find a comment on how we cannot have this.
> 
> Now I can't remember how important that all was for RT but I figured I'd
> post it and let other people stare at it for a bit.
> 




> +/*
> + * Does not allow usage from IRQ disabled, since we must be able to
> + * release IRQs to guarantee bounded hold time.

Actually we (-rt people) don't care if it's done with irq disabled, as
long is it was disabled via spin_lock_irq*() and not
raw_spin_lock_irq() or local_irq_save/disable().

Because in that case, in -rt, irqs would not be disabled. We could add a
check in -rt that makes sure that's the case. I would think this is OK
to call from an interrupt handler as long as it is a thread in -rt.

But, we can keep this restriction for now and change it if we find that
the restriction is preventing us from using it someplace where its
needed for -rt.

> + */
> +void __swake_up_all(struct swait_queue_head *q, unsigned int state)
> +{
> +	struct swait_queue *curr, *next;
> +	LIST_HEAD(tmp);
> +
> +	__swait_wakeup_debug(q, state);
> +
> +	if (!swait_active(q))
> +		return;
> +
> +	raw_spin_lock_irq(&q->lock);
> +	list_splice_init(&q->task_list, &tmp);
> +	while (!list_empty(&tmp)) {
> +		curr = list_first_entry(&tmp, typeof(curr), task_list);
> +
> +		wake_up_state(curr->task, state);
> +		list_del_init(&curr->task_list);

What happens if curr->task does not match state? We just removed it
from the list and never woke it up, didn't we?

-- Steve

> +
> +		if (list_empty(&tmp))
> +			break;
> +
> +		raw_spin_unlock_irq(&q->lock);
> +		raw_spin_lock_irq(&q->lock);
> +	}
> +	raw_spin_unlock_irq(&q->lock);
> +}
> +EXPORT_SYMBOL(__swake_up_all);
> +
--
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