linux-kernel - Re: [PATCH] nohz: Make tick_nohz_irq

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Date:	Thu, 21 Feb 2013 19:21:59 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] nohz: Make tick_nohz_irq_exit() irq safe

On Thu, 21 Feb 2013, Linus Torvalds wrote:

> On Wed, Feb 20, 2013 at 1:00 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >   */
> >  void irq_exit(void)
> >  {
> > +#ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
> > +       unsigned long flags;
> > +
> > +       local_irq_save(flags);
> > +#else
> > +       BUG_ON(!irqs_disabled();
> > +#endif
> 
> Guys, STOP DOING THIS!
> 
> Adding BUG_ON()'s just makes things much much much worse. There is
> *never* a reason to add a BUG_ON(). And doing it in an interrupt path
> is totally unacceptable. BUG_ON() makes it almost impossible to debug
> something, because you just killed the machine. So using BUG_ON() for
> "please notice this" is stupid as hell, because the most common end
> result is: "Oh, the machine just hung with no messages".
> 
> Make it WARN_ON_ONCE() if you absolutely have to let people know, but
> for something like this, why would you do even that?

This was a draft patch. I made it a WARN_ON_ONCE() already. 

We really want to enforce that irq_exit() is called with interrupts
disabled.

Thanks,

	tglx
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