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

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Date:   Wed, 15 Mar 2017 13:40:27 -0400 (EDT)
From:   Bob Peterson <rpeterso@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        cluster-devel <cluster-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: GFS2: pull request for high-priority bug

----- Original Message -----
| 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

Hi Linus,

Thanks. Yes, good ideas.
I see your point and I'll see if we can get that fixed up for the next merge window.

Bob Peterson