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

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Date:	Wed, 02 Feb 2011 14:51:57 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rostedt@...dmis.org
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	akpm@...ux-foundation.org, fweisbec@...il.com,
	mathieu.desnoyers@...icios.com, tglx@...utronix.de,
	peterz@...radead.org, rusty@...tcorp.com.au
Subject: Re: [PATCH 2/2] Tracepoints: Fix section alignment using pointer
 array

From: Steven Rostedt <rostedt@...dmis.org>
Date: Wed, 02 Feb 2011 13:06:15 -0500

> From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> 
> Make the tracepoints more robust, making them solid enough to handle compiler
> changes by not relying on anything based on compiler-specific behavior with
> respect to structure alignment. Implement an approach proposed by David Miller:
> use an array of const pointers to refer to the individual structures, and export
> this pointer array through the linker script rather than the structures per se.
> It will consume 32 extra bytes per tracepoint (24 for structure padding and 8
> for the pointers), but are less likely to break due to compiler changes.
> 
> 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.
> 
> The reason is that the "aligned" attribute only specify the _minimum_ alignment
> 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 as an
> array within each section, up-alignment cause NULL-pointer exceptions due to the
> extra unexpected padding.
> 
> (this patch applies on top of -tip)
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> LKML-Reference: <20110126222622.GA10794@...stal>
> CC: David Miller <davem@...emloft.net>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Ingo Molnar <mingo@...e.hu>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Rusty Russell <rusty@...tcorp.com.au>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by: David S. Miller <davem@...emloft.net>
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