ring-buffer: Set the right timestamp in the slow path of __rb_reserve_next()

In the slow path of __rb_reserve_next() a nested event(s) can happen
between evaluating the timestamp delta of the current event and updating
write_stamp via local_cmpxchg(); in this case the delta is not valid
anymore and it should be set to 0 (same timestamp as the interrupting
event), since the event that we are currently processing is not the last
event in the buffer.

Link: https://lkml.kernel.org/r/X8IVJcp1gRE+FJCJ@xps-13-7390
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lwn.net/Articles/831207
Fixes: a389d86f7fd0 ("ring-buffer: Have nested events still record running time stamp")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index bccaf88..35d91b2 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3287,11 +3287,11 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
                ts = rb_time_stamp(cpu_buffer->buffer);
                barrier();
  /*E*/         if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
-                   info->after < ts) {
+                   info->after < ts &&
+                   rb_time_cmpxchg(&cpu_buffer->write_stamp,
+                                   info->after, ts)) {
                        /* Nothing came after this event between C and E */
                        info->delta = ts - info->after;
-                       (void)rb_time_cmpxchg(&cpu_buffer->write_stamp,
-                                             info->after, ts);
                        info->ts = ts;
                } else {
                        /*