2 files changed, 12 insertions, 14 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 90cce0c74a03..f2b914de3f0c 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -167,30 +167,28 @@ struct perf_counter_mmap_page {
         /*
          * Bits needed to read the hw counters in user-space.
          *
-         * The index and offset should be read atomically using the seqlock:
-         *
-         *   __u32 seq, index;
-         *   __s64 offset;
+         *   u32 seq;
+         *   s64 count;
          *
          * again:
-         *   rmb();
          *   seq = pc->lock;
-         *
          *   if (unlikely(seq & 1)) {
          *     cpu_relax();
          *     goto again;
          *   }
          *
-         *   index = pc->index;
-         *   offset = pc->offset;
+         *   if (pc->index) {
+         *     count = pmc_read(pc->index - 1);
+         *     count += pc->offset;
+         *   } else
+         *     goto regular_read;
          *
-         *   rmb();
+         *   barrier();
          *   if (pc->lock != seq)
          *     goto again;
          *
-         * After this, index contains architecture specific counter index + 1,
-         * so that 0 means unavailable, offset contains the value to be added
-         * to the result of the raw timer read to obtain this counter's value.
+         * NOTE: for obvious reason this only works on self-monitoring
+         *       processes.
          */
         __u32   lock;                   /* seqlock for synchronization */
         __u32   index;                  /* hardware counter identifier */
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index f105a6e696c2..2a5d4f525567 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1340,13 +1340,13 @@ void perf_counter_update_userpage(struct perf_counter *counter)
          */
         preempt_disable();
         ++userpg->lock;
-        smp_wmb();
+        barrier();
         userpg->index = counter->hw.idx;
         userpg->offset = atomic64_read(&counter->count);
         if (counter->state == PERF_COUNTER_STATE_ACTIVE)
                 userpg->offset -= atomic64_read(&counter->hw.prev_count);
 
-        smp_wmb();
+        barrier();
         ++userpg->lock;
         preempt_enable();
 unlock: