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    SubjectRe: [PATCH 2.6.29.x - 2.6.31.1] module: fix __module_ref_addr()
    * Steven Rostedt (rostedt@goodmis.org) wrote:
    > This patch does not apply to 2.6.34-rc, and the code in upstream looks
    > to have been fixed. Should this go to stable?

    Yes. 2.6.34-rc does not have this issue anymore, but the patch is needed in
    -stable.

    Thanks,

    Mathieu

    >
    > -- Steve
    >
    >
    > On Sat, 2010-03-27 at 10:31 -0400, Mathieu Desnoyers wrote:
    > > __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


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