tracing: Only clear trace buffer on module unload if event was traced

Currently, when a module with events is unloaded, the trace buffer is
cleared. This is just a safety net in case the module might have some
strange callback when its event is outputted. But there's no reason
to reset the buffer if the module didn't have any of its events traced.

Add a flag to the event "call" structure called WAS_ENABLED and gets set
when the event is ever enabled, and this flag never gets cleared. When a
module gets unloaded, if any of its events have this flag set, then the
trace buffer will get cleared.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 0f1307a29fcf..9a7dc4bf1171 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -245,6 +245,9 @@ static int ftrace_event_enable_disable(struct ftrace_event_file *file,
                                 break;
                         }
                         file->flags |= FTRACE_EVENT_FL_ENABLED;
+
+                        /* WAS_ENABLED gets set but never cleared. */
+                        call->flags |= TRACE_EVENT_FL_WAS_ENABLED;
                 }
                 break;
         }
@@ -1626,12 +1629,13 @@ static void trace_module_remove_events(struct module *mod)
 {
         struct ftrace_module_file_ops *file_ops;
         struct ftrace_event_call *call, *p;
-        bool found = false;
+        bool clear_trace = false;
 
         down_write(&trace_event_mutex);
         list_for_each_entry_safe(call, p, &ftrace_events, list) {
                 if (call->mod == mod) {
-                        found = true;
+                        if (call->flags & TRACE_EVENT_FL_WAS_ENABLED)
+                                clear_trace = true;
                         __trace_remove_event_call(call);
                 }
         }
@@ -1648,9 +1652,9 @@ static void trace_module_remove_events(struct module *mod)
 
         /*
          * It is safest to reset the ring buffer if the module being unloaded
-         * registered any events.
+         * registered any events that were used.
          */
-        if (found)
+        if (clear_trace)
                 tracing_reset_current_online_cpus();
         up_write(&trace_event_mutex);
 }