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>

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.