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

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Date:	Fri, 22 Feb 2013 09:54:38 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Feb 21, 2013 at 10:21 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > This was a draft patch. I made it a WARN_ON_ONCE() already.
> 
> Ok, good.
> 
> I really wish we could just get rid of BUG_ON(). It was a bad 
> idea, and it makes it easy for people to do the wrong thing. 
> Sadly, we have tons of them.

So my old plan was to use a little bit of psychology.

Firstly, we could just turn BUG_ON() into a WARN() variant that 
emits:

  BUG: ...

while a WARN()ings emit:

  WARNING: ...

and then we could introduce a new primitive:

  CRASH_ON();

which would be used in the (few) places that really, really 
cannot continue sanely and need to crash the box.

This naming alone would inhibit its use through two channels:

 - Putting the word 'CRASH' into your code feels risky, 
   dissonant and wrong (perfect code does not crash) and thus 
   needs conscious frontal lobe effort to justify it - while 
   BUG_ON() really feels more like a harmless assert to most 
   kernel developers, which is in our muscle memory through 
   years training.

 - CRASH_ON() takes one character more typing than WARN_ON(), 
   and we know good kernel developers are fundamentally lazy.

[ This is an arguably lazy plan that does not involve changing 
  the 10,000+ BUG_ON() call sites and does not involve the
  re-training of thousands of mis-trained kernel developers who 
  introduced over 900 new BUG_ON()s in the v3.7->v3.8 cycle 
  alone (!). ]

So while I don't think we can win the war against BUG_ON(), I 
think we can fight the lazy general's war: turn the enemy over 
to our side and declare victory?

Thanks,

	Ingo
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