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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>
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>
This commit is contained in:
Peter Zijlstra 2009-04-06 11:44:59 +02:00 committed by Ingo Molnar
parent 92f22a3865
commit a2e87d06dd
1 changed files with 10 additions and 14 deletions
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24
include/linux/perf_counter.h
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@ -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.