ring-buffer: Always check to put back before stamp when crossing pages

commit 68e10d5ff512b503dcba1246ad5620f32035e135 upstream.

The current ring buffer logic checks to see if the updating of the event
buffer was interrupted, and if it is, it will try to fix up the before stamp
with the write stamp to make them equal again. This logic is flawed, because
if it is not interrupted, the two are guaranteed to be different, as the
current event just updated the before stamp before allocation. This
guarantees that the next event (this one or another interrupting one) will
think it interrupted the time updates of a previous event and inject an
absolute time stamp to compensate.

The correct logic is to always update the timestamps when traversing to a
new sub buffer.

Cc: stable@vger.kernel.org
Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running time stamp")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 368d7f2e6d5099a1faad77727464a79ef5432a32..0ebbf18a8fb51ecc621d0919956d845ca6ade78e 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3234,14 +3234,12 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 
        /* See if we shot pass the end of this buffer page */
        if (unlikely(write > BUF_PAGE_SIZE)) {
-               if (tail != w) {
-                       /* before and after may now different, fix it up*/
-                       b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
-                       a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
-                       if (a_ok && b_ok && info->before != info->after)
-                               (void)rb_time_cmpxchg(&cpu_buffer->before_stamp,
-                                                     info->before, info->after);
-               }
+               /* before and after may now different, fix it up*/
+               b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
+               a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
+               if (a_ok && b_ok && info->before != info->after)
+                       (void)rb_time_cmpxchg(&cpu_buffer->before_stamp,
+                                             info->before, info->after);
                return rb_move_tail(cpu_buffer, tail, info);
        }