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    Subject[PATCH 5/7] perf_counter: fix the output lock
    Use -1 instead of 0 as unlocked, since 0 is a valid cpu number.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/perf_counter.c | 39 ++++++++++++++++++++++++++++++++++-----
    1 file changed, 34 insertions(+), 5 deletions(-)

    Index: linux-2.6/kernel/perf_counter.c
    ===================================================================
    --- linux-2.6.orig/kernel/perf_counter.c
    +++ linux-2.6/kernel/perf_counter.c
    @@ -1409,6 +1417,7 @@ static int perf_mmap_data_alloc(struct p
    }

    data->nr_pages = nr_pages;
    + atomic_set(&data->lock, -1);

    rcu_assign_pointer(counter->data, data);

    @@ -1755,7 +1764,7 @@ static void perf_output_lock(struct perf
    if (in_nmi() && atomic_read(&data->lock) == cpu)
    return;

    - while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
    + while (atomic_cmpxchg(&data->lock, -1, cpu) != -1)
    cpu_relax();

    handle->locked = 1;
    @@ -1784,7 +1793,7 @@ again:
    * NMI can happen here, which means we can miss a done_head update.
    */

    - cpu = atomic_xchg(&data->lock, 0);
    + cpu = atomic_xchg(&data->lock, -1);
    WARN_ON_ONCE(cpu != smp_processor_id());

    /*
    @@ -1794,7 +1803,7 @@ again:
    /*
    * Since we had it locked, we can lock it again.
    */
    - while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
    + while (atomic_cmpxchg(&data->lock, -1, cpu) != -1)
    cpu_relax();

    goto again;
    --



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