[patch] ext2fs freeze/corruption on amd64

[Jaakko Heinonen]

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.

-- 
Jaakko