linux-kernel - Re: [PATCH] x86: remove unneeded endless loop in BUG()

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Date:	Thu, 19 Feb 2009 21:32:21 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: remove unneeded endless loop in BUG()


* H. Peter Anvin <hpa@...or.com> wrote:

> Ingo Molnar wrote:
>>
>> the problem is that the DO_BUG() will generate the u2d instruction into 
>> a random place where GCC puts it. It certainly wont be in the place 
>> where the __bug_table logic above expects it.
>>
>> The result will be cryptic crashes instead of a clean BUG message 
>> assert.
>>
>
> I went and talked to H.J. Lu about this.
>
> He said __builtin_trap(); is functionally treated as an asm volatile,  
> and that it is most likely impossible that gcc could do anything wrong  
> here (he did specifically state that nothing can move across the asm  
> volatile, and there are no data dependencies between the asm volatile  
> and the __builtin_trap).
>
> He also agreed that the right way to do this is __builtin_not_reached(),  
> and I promised to submit a feature request for this for a future version  
> of gcc.
>
> Given that, I would suggest we back out the patch, and that when  
> __builtin_not_reached(); is supported, we can simply do:
>
> #if __GNUC__ is recent enough
> # define not_reached() __builtin_not_reached()
> #else
> # define not_reached() for(;;)
> #endif
>
> OK?

Yeah, sounds good!

	Ingo
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