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>

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 009a655..2fc1602 100644 (file)
--- 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 */

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index cd55144..9c2ddfb 100644 (file)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -87,10 +87,31 @@ again:
                goto out;
 
        /*
-        * Publish the known good head. Rely on the full barrier implied
-        * by atomic_dec_and_test() order the rb->head read and this
-        * write.
+        * Since the mmap() consumer (userspace) can run on a different CPU:
+        *
+        *   kernel                             user
+        *
+        *   READ ->data_tail                   READ ->data_head
+        *   smp_mb()   (A)                     smp_rmb()       (C)
+        *   WRITE $data                        READ $data
+        *   smp_wmb()  (B)                     smp_mb()        (D)
+        *   STORE ->data_head                  WRITE ->data_tail
+        *
+        * Where A pairs with D, and B pairs with C.
+        *
+        * I don't think A needs to be a full barrier because we won't in fact
+        * write data until we see the store from userspace. So we simply don't
+        * issue the data WRITE until we observe it. Be conservative for now.
+        *
+        * OTOH, D needs to be a full barrier since it separates the data READ
+        * from the tail WRITE.
+        *
+        * For B a WMB is sufficient since it separates two WRITEs, and for C
+        * an RMB is sufficient since it separates two READs.
+        *
+        * See perf_output_begin().
         */
+       smp_wmb();
        rb->user_page->data_head = head;
 
        /*
@@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output_handle *handle,
                 * Userspace could choose to issue a mb() before updating the
                 * tail pointer. So that all reads will be completed before the
                 * write is issued.
+                *
+                * See perf_output_put_handle().
                 */
                tail = ACCESS_ONCE(rb->user_page->data_tail);
-               smp_rmb();
+               smp_mb();
                offset = head = local_read(&rb->head);
                head += size;
                if (unlikely(!perf_output_space(rb, tail, offset, head)))