linux-kernel - Re: [RFC PATCH] Tracepoints: fix section alignment using pointer array

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Date:	Mon, 31 Jan 2011 13:48:27 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] Tracepoints: fix section alignment using pointer array

On Thu, 27 Jan 2011 08:56:22 am Mathieu Desnoyers wrote:
> 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.

Hmm, that assumption is used in module parameters too, so we already rely on 
the toolchain not to over-pad.

Perhaps we should fix that too, or wait until it explodes?

Cheers,
Rusty.
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