Date: Wed, 23 May 2012 14:54:58 +0800
From: Yong Zhang <yong.zhang0@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Christophe Huriaux <c.huriaux@gmail.com>,
        Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, Andreas Mohr <andi@lisas.de>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] genirq: don't sync irq thread if current happen to be
 the very irq thread
Message-ID: <20120523065458.GA24852@zhy>
Reply-To: Yong Zhang <yong.zhang0@gmail.com>
References: <CAEOomG9NEoQKqp=pMx0NpzGaFpHBimALwhemnZL4F-8g7XURkA@mail.gmail.com>
 <20120509174931.GB31844@pengutronix.de>
 <CAEOomG9FZD9TEQZZKhCrNrq8SNXi3otWB2mDX69i3tWh5YQenw@mail.gmail.com>
 <20120520052731.GA3864@zhy>
 <20120520121926.GA24585@zhy>
 <alpine.LFD.2.02.1205221548010.3231@ionos>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <alpine.LFD.2.02.1205221548010.3231@ionos>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1988
Lines: 61

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/