[patch] ext2fs freeze/corruption on amd64

[Kostik Belousov]

On Sat, Jan 03, 2009 at 08:21:08PM +0200, Jaakko Heinonen wrote:
> 
> Hi,
> 
> I can quite easily reproduce a hard freeze while writing to an ext2fs
> filesystem on amd64. I can reliably reproduce it by building a kernel
> with "make -j2" on ext2fs.
> 
> This has been reported by other people too:
> 
> http://lists.freebsd.org/pipermail/freebsd-fs/2008-March/004504.html
> http://lists.freebsd.org/pipermail/freebsd-bugs/2008-January/027555.html
> http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/119529
> http://www.freebsd.org/cgi/query-pr.cgi?pr=amd64/99561
> 
> I tracked the problem down to find_next_zero_bit() in ext2_bitops.h.
> The bug doesn't affect i386 because it uses asm bitops from
> i386-bitops.h. (There's also sparc64-bitops.h file but it's not used
> anywhere.)
> 
> The problem is that if the offset (ofs) has one or more of it's five low
> bits set those bits are incorrectly re-added to the result. With
> following patch applied I can't reproduce the problem.
> 
> %%%
> Index: sys/gnu/fs/ext2fs/ext2_bitops.h
> ===================================================================
> --- sys/gnu/fs/ext2fs/ext2_bitops.h	(revision 186639)
> +++ sys/gnu/fs/ext2fs/ext2_bitops.h	(working copy)
> @@ -84,7 +84,7 @@ find_next_zero_bit(void *data, size_t sz
>  		mask = ~0U << (ofs & 31);
>  		bit = *p | ~mask;
>  		if (bit != ~0U)
> -			return (ffs(~bit) + ofs - 1);
> +			return (ffs(~bit) + (ofs & ~31U) - 1);
>  		p++;
>  		ofs = (ofs + 31U) & ~31U;
>  	}
> %%%
> 
> I hope that someone would commit this or similar fix. It might be also
> reasonable to delete the unused sparc64-bitops.h file.

Please note that find_next_zero_bit() uses gcc extension to perform
void-pointer arithmetic. Also, at the +2 line, it performs unaligned
access to the uint32_t entities.
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