markers: comment marker_synchronize_unregister() on data dependency

Add document and comments on marker_synchronize_unregister(): it
should be called before freeing resources that the probes depend on.

Based on comments from Lai Jiangshan and Mathieu Desnoyers.
Signed-off-by: Wu Fengguang <wfg@linux.intel.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/markers.txt

@@ -51,11 +51,16 @@ to call) for the specific marker through marker_probe_register() and can be
 activated by calling marker_arm(). Marker deactivation can be done by calling
 marker_disarm() as many times as marker_arm() has been called. Removing a probe
 is done through marker_probe_unregister(); it will disarm the probe.
-marker_synchronize_unregister() must be called before the end of the module exit
-function to make sure there is no caller left using the probe. This, and the
-fact that preemption is disabled around the probe call, make sure that probe
-removal and module unload are safe. See the "Probe example" section below for a
-sample probe module.
+
+marker_synchronize_unregister() must be called between probe unregistration and
+the first occurrence of
+- the end of module exit function,
+  to make sure there is no caller left using the probe;
+- the free of any resource used by the probes,
+  to make sure the probes wont be accessing invalid data.
+This, and the fact that preemption is disabled around the probe call, make sure
+that probe removal and module unload are safe. See the "Probe example" section
+below for a sample probe module.
 
 The marker mechanism supports inserting multiple instances of the same marker.
 Markers can be put in inline functions, inlined static functions, and

include/linux/marker.h

@@ -211,8 +211,10 @@ extern void *marker_get_private_data(const char *name, marker_probe_func *probe,
 
 /*
  * marker_synchronize_unregister must be called between the last marker probe
- * unregistration and the end of module exit to make sure there is no caller
- * executing a probe when it is freed.
+ * unregistration and the first one of
+ * - the end of module exit function
+ * - the free of any resource used by the probes
+ * to ensure the code and data are valid for any possibly running probes.
  */
 #define marker_synchronize_unregister() synchronize_sched()