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    Subject[PATCH 01/15] perf_counter: update mmap() counter read, take 2
    Update the userspace read method.

    Paul noted that:
    - userspace cannot observe ->lock & 1 on the same cpu.
    - we need a barrier() between reading ->lock and ->index
    to ensure we read them in that prticular order.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    include/linux/perf_counter.h | 24 ++++++++++--------------
    1 file changed, 10 insertions(+), 14 deletions(-)

    Index: linux-2.6/include/linux/perf_counter.h
    ===================================================================
    --- linux-2.6.orig/include/linux/perf_counter.h
    +++ linux-2.6/include/linux/perf_counter.h
    @@ -170,22 +170,18 @@ struct perf_counter_mmap_page {
    * u32 seq;
    * s64 count;
    *
    - * again:
    - * seq = pc->lock;
    - * if (unlikely(seq & 1)) {
    - * cpu_relax();
    - * goto again;
    - * }
    + * do {
    + * seq = pc->lock;
    *
    - * if (pc->index) {
    - * count = pmc_read(pc->index - 1);
    - * count += pc->offset;
    - * } else
    - * goto regular_read;
    + * barrier()
    + * if (pc->index) {
    + * count = pmc_read(pc->index - 1);
    + * count += pc->offset;
    + * } else
    + * goto regular_read;
    *
    - * barrier();
    - * if (pc->lock != seq)
    - * goto again;
    + * barrier();
    + * } while (pc->lock != seq);
    *
    * NOTE: for obvious reason this only works on self-monitoring
    * processes.
    --



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