Messages in this thread Patch in this message | | | Date | Mon, 06 Apr 2009 11:44:59 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 01/15] perf_counter: update mmap() counter read, take 2 |
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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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