linux-kernel - Re: GFS2: pull request for high-priority bug

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Date:   Wed, 15 Mar 2017 10:21:28 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Bob Peterson <rpeterso@...hat.com>,
        Andreas Gruenbacher <agruenba@...hat.com>
Cc:     cluster-devel <cluster-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: GFS2: pull request for high-priority bug

On Wed, Mar 15, 2017 at 7:32 AM, Bob Peterson <rpeterso@...hat.com> wrote:
>
> Andreas Gruenbacher (1):
>       gfs2: Avoid alignment hole in struct lm_lockname

So I've pulled this because I think it fixes a real bug, but honestly
I think it's the wrong fix.

Marking that lm_lockname structure "packed, aligned(4)" means that the
compiler will now think that the 64-bit fields in it may be unaligned
- including on architectures where that can be very expensive and the
compiler now might generate stupid unaligned instruction sequences to
load those values.

So the *correct* fix, I think, would have been:

 - add a comment about not having holes in the struct due to the hashing

 - sort the fields by size (so "ln_number" first, then "ln_sbd", then "ln_type")

 - use offsetofend(struct lm_lockname, ln_type) instead of sizeof() when hashing

which avoids the "possibly generate garbage code" issue due to the
quick-and-dirty one-liner approach.

Hmm?

                 Linus