Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull perf fixes from Ingo Molnar:
 "Two fixes:

   - Fix 'NMI handler took too long to run' false positives

     [ Genuine NMI overhead speedups will come for v3.13, this commit
       only fixes a measurement bug ]

   - Fix perf ring-buffer missed barrier causing (rare) ring-buffer data
     corruption on ppc64"

* 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  perf/x86: Fix NMI measurements
  perf: Fix perf ring buffer memory ordering

1 files changed, 7 insertions, 5 deletions

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 009a655a5d35..2fc1602e23bb 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -456,13 +456,15 @@ struct perf_event_mmap_page {
         /*
          * Control data for the mmap() data buffer.
          *
-         * User-space reading the @data_head value should issue an rmb(), on
-         * SMP capable platforms, after reading this value -- see
-         * perf_event_wakeup().
+         * User-space reading the @data_head value should issue an smp_rmb(),
+         * after reading this value.
          *
          * When the mapping is PROT_WRITE the @data_tail value should be
-         * written by userspace to reflect the last read data. In this case
-         * the kernel will not over-write unread data.
+         * written by userspace to reflect the last read data, after issueing
+         * an smp_mb() to separate the data read from the ->data_tail store.
+         * In this case the kernel will not over-write unread data.
+         *
+         * See perf_output_put_handle() for the data ordering.
          */
         __u64   data_head;              /* head in the data section */
         __u64   data_tail;              /* user-space written tail */