linux-kernel - Re: [PATCH 2/2] Tracepoints: Fix section alignment using pointer array

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Date:	Wed, 2 Feb 2011 14:32:09 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 2/2] Tracepoints: Fix section alignment using pointer
	array

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Wed, 2011-02-02 at 13:31 -0500, Mathieu Desnoyers wrote:
> 
> > > History:
> > > 
> > > commit 7e066fb870fcd1025ec3ba7bbde5d541094f4ce1 added the aligned(32) type and
> > > variable attribute to the tracepoint structures to deal with gcc happily
> > > aligning statically defined structures on 32-byte multiples.
> > > 
> > > commit 15e3540ce2159705f18fad6147ffedf04445ad64 tried to use a 8-byte alignment
> > > for tracepoint structures by applying both the variable and type attribute to
> > > tracepoint structures definitions and declarations. It worked fine with gcc
> > > 4.5.1, but broke with gcc 4.4.4 and 4.4.5.
> > 
> > Small nit: this reference to commit 15e3540ce2159705f18fad6147ffedf04445ad64
> > should be changed to a non-commit-id-related explanation, because the commit has
> > been rolled back from -tip.
> 
> OK, I just did a git commit --amend to fix it. Here's the new text:
> 
> 
>     commit 7e066fb870fcd1025ec3ba7bbde5d541094f4ce1 added the aligned(32) type a
>     variable attribute to the tracepoint structures to deal with gcc happily
>     aligning statically defined structures on 32-byte multiples.
>     
>     One attempt was to use a 8-byte alignment for tracepoint structures by apply
>     both the variable and type attribute to tracepoint structures definitions an
>     declarations. It worked fine with gcc 4.5.1, but broke with gcc 4.4.4 and 4.
>     
>     The reason is that the "aligned" attribute only specify the _minimum_ alignm
>     for a structure, leaving both the compiler and the linker free to align on
>     larger multiples. Because tracepoint.c expects the structures to be placed a
>     array within each section, up-alignment cause NULL-pointer exceptions due to
>     extra unexpected padding.
> 
> This version has just been pushed. I just changed the wording to leave
> out the reference to the reverted commit.

Looks good!

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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