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| author | Peter Zijlstra <a.p.zijlstra@chello.nl> | 2009-04-06 11:44:59 +0200 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2009-04-07 10:48:54 +0200 |
| commit | a2e87d06ddbe6e6fdb8d6d2e5e985efe4efb07dd (patch) | |
| tree | af0fdeb24c4e957d4e87a50ba358e5fdba6f7600 /include/linux/perf_counter.h | |
| parent | 92f22a3865abe87eea2609a6f8e5be5123f7ce4f (diff) | |
| download | lwn-a2e87d06ddbe6e6fdb8d6d2e5e985efe4efb07dd.tar.gz lwn-a2e87d06ddbe6e6fdb8d6d2e5e985efe4efb07dd.zip | |
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>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090406094517.368446033@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'include/linux/perf_counter.h')
| -rw-r--r-- | include/linux/perf_counter.h | 24 |
1 files changed, 10 insertions, 14 deletions
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h index f2b914de3f0c..e22ab47a2f41 100644 --- a/include/linux/perf_counter.h +++ b/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. |