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

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Message-ID: <CA+55aFwTFOsRTX3pHeZ+664rAms-Liwco3scEPJum6=C3BCoEQ@mail.gmail.com>
Date:	Thu, 21 Feb 2013 09:53:48 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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 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?

                          Linus
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