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 <[email protected]>
Reviewed-by: Mathieu Desnoyers <[email protected]>
Reviewed-by: Lai Jiangshan <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
diff --git a/Documentation/markers.txt b/Documentation/markers.txt
index 6d275e4..d2b3d0e 100644
--- a/Documentation/markers.txt
+++ b/Documentation/markers.txt
@@ -51,11 +51,16 @@
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
diff --git a/include/linux/marker.h b/include/linux/marker.h
index 34c14bc..b85e74c 100644
--- a/include/linux/marker.h
+++ b/include/linux/marker.h
@@ -211,8 +211,10 @@
/*
* 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()
