linux-kernel - Re: [PATCH v2] genirq: don't sync irq thread if current happen to be the very irq thread

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Date:	Wed, 23 May 2012 14:54:58 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Christophe Huriaux <c.huriaux@...il.com>,
	Uwe Kleine-Koenig <u.kleine-koenig@...gutronix.de>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andreas Mohr <andi@...as.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] genirq: don't sync irq thread if current happen to be
 the very irq thread

On Tue, May 22, 2012 at 03:50:38PM +0200, Thomas Gleixner wrote:
> On Sun, 20 May 2012, Yong Zhang wrote:
> > On Sun, May 20, 2012 at 01:27:31PM +0800, Yong Zhang wrote:
> > > --- a/kernel/irq/manage.c
> > > +++ b/kernel/irq/manage.c
> > > @@ -41,6 +41,7 @@ early_param("threadirqs", setup_forced_irqthreads);
> > >  void synchronize_irq(unsigned int irq)
> > >  {
> > >  	struct irq_desc *desc = irq_to_desc(irq);
> > > +	struct irqaction *action = desc->action;
> > 
> > Bad time for dereferencing *action.
> 
> You meant dereferencing *desc :)

Ah, yes :)

>  
> >  	/*
> >  	 * We made sure that no hardirq handler is running. Now verify
> >  	 * that no threaded handlers are active.
> > +	 * But for theaded irq, we don't sync if current happens to be
> > +	 * the irq thread; otherwise we could deadlock.
> >  	 */
> > +	action = desc->action;
> 
> And dereferencing action w/o being protected by desc->lock is buggy.
> 
> +	while (action) {
> > +		if (action->thread && action->thread == current)
> > +			return;
> > +		action = action->next;
> > +	}
> > +
> 
> Aside of that I really do not like that change. It'll hide real
> deadlocks when disable_irq() is called from the interrupt handler.
> 
> Also this will not cure all problems of that MMC driver on RT or with
> forced threaded interrupts.
> 
> Assume that tasklet code runs from the softirq thread so it will
> schedule when desc->threads_active > 0. This will trigger a
> "scheduling while atomic" warning.

Yes.

> 
> The irq_enable/disable dance in that driver is amazing. I have no time
> at the moment to grok the logic behind this, but it bet this can be
> done way simpler and less horrible.

I'll reconsider this issue and try to find the simpler way.

Thanks,
Yong
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