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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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