linux-kernel - Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86

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Date:	Tue, 30 Jun 2009 13:10:54 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Mikael Pettersson <mikpe@...uu.se>,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Matthew Wilcox <matthew@....cx>,
	Grant Grundler <grundler@...isc-linux.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org
Subject: Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit
 x86



On Tue, 30 Jun 2009, H. Peter Anvin wrote:
> 
> By process of elimination, the culprit must be round_up(), which reveals
> that the macro definition of round_up() has a *very* sublte behavior
> with mixed types:
> 
> #define round_up(x, y) (((x) + (y) - 1) & ~((y) - 1))
> 
> ram_alignment() returns unsigned long, which becomes (y).  This means
> that the mask word on the right hand of the & gets truncated to 32 bits
> *before* the masking happens -- since ((y) - 1) is still unsigned long,
> inverting it will not set bits [63..32] to on.

Good catch.

Also, this shows another bug in the #define: it evaluates 'y' twice, which 
is a no-no for something that _looks_ like a function.

> I think this macro is actively dangerous.  Better would be:
> 
> ({ __typeof__(x) __mask = (y)-1;  ((x)+__mask) & ~__mask; })

Yes. Please make it so.

> The deep irony in this is that in our particular case is perhaps that
> align_up(x,y)-1 is the same thing as x | (y-1) which would have avoided
> the problem...

I don't know how deep that irony is, but I do agree that maybe we should 
do that simplification too. In addition to fixing round_up() to not bite 
future generations in the ass.

		Linus
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