perf: Fix perf ring buffer memory ordering

The PPC64 people noticed a missing memory barrier and crufty old
comments in the perf ring buffer code. So update all the comments and
add the missing barrier.

When the architecture implements local_t using atomic_long_t there
will be double barriers issued; but short of introducing more
conditional barrier primitives this is the best we can do.

Reported-by: Victor Kaplansky <victork@il.ibm.com>
Tested-by: Victor Kaplansky <victork@il.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: michael@ellerman.id.au
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: anton@samba.org
Cc: benh@kernel.crashing.org
Link: http://lkml.kernel.org/r/20131025173749.GG19466@laptop.lan
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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 */