[PATCH 2.6.29.x - 2.6.31.1] module: fix __module_ref_addr()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

__module_ref_addr() should use per_cpu_ptr() to obfuscate the pointer
(RELOC_HIDE is needed for per cpu pointers).

This non-standard per-cpu pointer use has been introduced by commit
720eba31f47aeade8ec130ca7f4353223c49170f

It causes a NULL pointer exception on some configurations when CONFIG_TRACING is
enabled on 2.6.33. This patch fixes the problem (acknowledged by Randy who
reported the bug).

It did not appear to hurt previously because most of the accesses were done
through local_inc, which probably obfuscated the access enough that no compiler
optimizations were done. But with local_read() done when CONFIG_TRACING is
active, this becomes a problem. Non-CONFIG_TRACING is probably affected as well
(module.c contains local_set and local_read that use __module_ref_addr()), but I
guess nobody noticed because we've been lucky enough that the compiler did not
generate the inappropriate optimization pattern there.

This patch should be queued for the 2.6.29.x through 2.6.33.x stable branches.
(tested on 2.6.33.1 x86_64)

The __module_ref_addr() problem disappears in 2.6.34-rc kernels because these
percpu accesses were re-factored.

It makes me wonder about other non-standard uses of per_cpu_offset: there is one
in module.c and two in lockdep.c, which are still in 2.6.34-rc. This should
probably be fixed by the code authors in separate patches.

lockdep.c: commit 8e18257d29238311e82085152741f0c3aa18b74d
module.c: commit 6b588c18f8dacfa6d7957c33c5ff832096e752d3

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
CC: Eric Dumazet <dada1@cosmosbay.com>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Tejun Heo <tj@kernel.org>
CC: Ingo Molnar <mingo@elte.hu>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Greg Kroah-Hartman <gregkh@suse.de>
CC: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/module.h |    2 +-
 kernel/module.c        |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

Index: linux-2.6-lttng/include/linux/module.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/module.h	2010-03-25 11:01:53.000000000 -0400
+++ linux-2.6-lttng/include/linux/module.h	2010-03-25 11:01:59.000000000 -0400
@@ -467,7 +467,7 @@ void symbol_put_addr(void *addr);
 static inline local_t *__module_ref_addr(struct module *mod, int cpu)
 {
 #ifdef CONFIG_SMP
-	return (local_t *) (mod->refptr + per_cpu_offset(cpu));
+	return (local_t *) per_cpu_ptr(mod->refptr, cpu);
 #else
 	return &mod->ref;
 #endif

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