linux-kernel - Re: Definition of BUG on x86

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Date:	Thu, 19 Feb 2009 13:38:02 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Definition of BUG on x86

Ingo Molnar píše v Čt 19. 02. 2009 v 13:22 +0100:
> * Petr Tesarik <ptesarik@...e.cz> wrote:
> 
> > Ingo Molnar píše v Čt 19. 02. 2009 v 13:10 +0100:
> > > * Petr Tesarik <ptesarik@...e.cz> wrote:
> > > 
> > > > So, the only method I could invent was using gas macros. It 
> > > > works but is quite ugly, because it relies on the actual 
> > > > assembler instruction which is generated by the compiler. Now, 
> > > > AFAIK gcc has always translated "for(;;)" into a jump to self, 
> > > > and that with any conceivable compiler options, but I don't 
> > > > know anything about Intel cc.
> > > 
> > > > +static inline __noreturn void discarded_jmp(void)
> > > > +{
> > > > +	asm volatile(".macro jmp target\n"
> > > > +		     "\t.purgem jmp\n"
> > > > +		     ".endm\n");
> > > > +	for (;;) ;
> > > > +}
> > > 
> > > hm, that's very fragile.
> > > 
> > > Why not just:
> > > 
> > >  static inline __noreturn void x86_u2d(void)
> > >  {
> > > 	asm volatile("u2d\n");
> > >  }
> > > 
> > > If GCC emits a bogus warning about _that_, then it's a bug in 
> > > the compiler that should be fixed.
> > 
> > I wouldn't call it a bug. The compiler has no idea about what 
> > the inline assembly actualy does. So it cannot recognize that 
> > the ud2 instruction does not return (which BTW might not even 
> > be the case, depending on the implementation of the Invalid 
> > Opcode exception).
> 
> No, i'm not talking about the inline assembly.
> 
> I'm talking about the x86_u2d() _inline function_, which has 
> the __noreturn attribute.
> 
> Shouldnt that be enough to tell the compiler that it ... wont 
> return?

Nope, that's not how it works.

You _may_ specify a noreturn attribute to any function (and GCC will
honour it AFAICS), but if GCC _thinks_ that the function does return, it
will issue the above-mentioned warning:

/usr/src/linux-2.6/arch/x86/include/asm/bug.h:10: warning: 'noreturn' function does return

And that's what your function will do. :-(

Yes, I also thinks that this behaviour is counter-intuitive. Besides, I
haven't found a gcc switch to turn this warning off, which would be my
next recommendation, since the GCC heuristics is broken, of course.

Petr Tesarik


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