linux-kernel - Re: [PATCH] taskstats: Use better ifdef for alignment

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Message-Id: <20101229213243.891b0db5.akpm@linux-foundation.org>
Date:	Wed, 29 Dec 2010 21:32:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Dan Carpenter <error27@...il.com>, balbir@...ux.vnet.ibm.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] taskstats: Use better ifdef for alignment

On Thu, 30 Dec 2010 00:26:34 -0500 Jeff Mahoney <jeffm@...e.com> wrote:

> On 12/29/2010 07:14 PM, Andrew Morton wrote:
> > On Wed, 29 Dec 2010 19:12:08 -0500 Jeff Mahoney <jeffm@...e.com> wrote:
> > 
> >>  Commit 4be2c95d added a null field to align the taskstats structure but
> >>  the discussion centered around ia64. The issue exists on other platforms
> >>  with inefficient unaligned access and adding them piecemeal would be
> >>  an unmaintainable mess.
> >>
> >>  This patch uses Dave Miller's suggestion of using a combination of
> >>  CONFIG_64BIT && !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to determine
> >>  whether alignment is needed.
> >>
> >>  Note that this will cause breakage on those platforms with applications
> >>  like iotop which had hard-coded offsets into the packet to access the
> >>  taskstats structure.
> > 
> > That seems a very good reason to not apply the patch.
> > 
> > Tell us more, please...
> 
> I don't want to rehash the same discussion

Please do so.  That discussion went on for a long time over many emails
and multiple iterations of the patch.  I personally have forgotten the
reasoning and if I could remember it, I wouldn't remember which version
of the patch it applied to.

Applying a patch which is *known* to break *known* userspace
applications is a quite extraordinary thing to do.  We owe it to people
to fully explain the reasoning.

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