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    SubjectRe: [Kgdb-bugreport] [PATCH 4/5] kgdb: Use atomic operators whichuse barriers
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    Hi Jason,

    > Here is the revised patch, which is going into the kgdb-fixes branch.
    >
    > Thanks,
    > Jason.
    >
    > --
    > From: Jason Wessel <jason.wessel@windriver.com>
    > Subject: [PATCH] kgdb: Use atomic operators which use barriers
    >
    > The cpu_relax() does not mandate that there is an smp memory barrier.
    > As a result on the arm smp architecture the kernel debugger can hang
    > on entry from time to time, as shown by the kgdb regression tests.
    >
    > The solution is simply to use the atomic operators which include a
    > proper smp memory barrier, instead of using atomic_set().
    >
    > Tested-by: Will Deacon <will.deacon@arm.com>
    > Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
    > ---
    > kernel/kgdb.c | 15 +++++++--------
    > 1 file changed, 7 insertions(+), 8 deletions(-)
    >
    > --- a/kernel/kgdb.c
    > +++ b/kernel/kgdb.c
    > @@ -1379,8 +1379,7 @@ acquirelock:
    > * Make sure the above info reaches the primary CPU before
    > * our cpu_in_kgdb[] flag setting does:
    > */
    > - smp_wmb();
    > - atomic_set(&cpu_in_kgdb[cpu], 1);
    > + atomic_inc(&cpu_in_kgdb[cpu]);

    As Dmitry pointed out here:

    http://lkml.org/lkml/2010/4/8/214

    This bit of code looks broken, especially since the comment has been left
    alone by the patch. I think commit ae6bf53e should be reverted because
    semantically all it does is remove the smp_wmb() above.

    Please let me know what you think,

    Will





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