linux-kernel - Re: [xfs-masters] Re: filesystem corruption on xfs after 2.6.25-rc1 (bisected, powerpc related?)

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Message-Id: <1204049492.13162.265.camel@johannes.berg>
Date:	Tue, 26 Feb 2008 19:11:32 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Gaudenz Steinlin <gaudenz@...iologie.ch>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Christoph Hellwig <hch@....de>,
	xfs-masters@....sgi.com, Eric Sandeen <sandeen@...deen.net>,
	xfs@....sgi.com,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [xfs-masters] Re: filesystem corruption on xfs after
	2.6.25-rc1 (bisected, powerpc related?)

> I debuged this a bit further by testing the 4 changed functions
> individually. The problem only occurs with the new version of
> xfs_lowbit64. 

Eh, uh, of course. Now that I look at that code it becomes obvious.

find_first_bit() works on unsigned longs, not 64-bit quantities, so
find_first_bit((unsigned long *)&t, 64) isn't equivalent to finding the
lowest bit set in a 64-bit quantity.

Think of the memory layout of a 64-bit word:

LE:	low 32 bits | high 32 bits
BE:	high 32 bits | low 32 bits

Take a look at the start of include/asm-powerpc/bitops.h, and note how
bitops don't define the memory layout at all :)

So find_first_bit(&t, 64) on BE will give you the number of the first
bit of the 32-bit rotated quantity, ie. of ((t<<32) | (t>>32)).

The problem doesn't happen with highbit64 because fls64 was specifically
coded for this purpose.

You really need to keep xfs_lowbit64 defined as it was before, or, maybe
even better, define ffs64 in parallel to fls64.

johannes

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