Messages in this thread | | | Date | Wed, 02 Feb 2011 14:51:57 -0800 (PST) | Subject | Re: [PATCH 2/2] Tracepoints: Fix section alignment using pointer array | From | David Miller <> |
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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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