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    SubjectRe: [PATCH 2/2] Tracepoints: Fix section alignment using pointer array
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    From: Steven Rostedt <rostedt@goodmis.org>
    Date: Wed, 02 Feb 2011 13:06:15 -0500

    > From: Mathieu Desnoyers <mathieu.desnoyers@efficios.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@efficios.com>
    > LKML-Reference: <20110126222622.GA10794@Krystal>
    > CC: David Miller <davem@davemloft.net>
    > CC: Frederic Weisbecker <fweisbec@gmail.com>
    > CC: Ingo Molnar <mingo@elte.hu>
    > CC: Thomas Gleixner <tglx@linutronix.de>
    > CC: Andrew Morton <akpm@linux-foundation.org>
    > CC: Peter Zijlstra <peterz@infradead.org>
    > CC: Rusty Russell <rusty@rustcorp.com.au>
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

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


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