tracepoint: Give priority to probes of tracepoints

In order to guarantee that a probe will be called before other probes that
are attached to a tracepoint, there needs to be a mechanism to provide
priority of one probe over the others.

Adding a prio field to the struct tracepoint_func, which lets the probes be
sorted by the priority set in the structure. If no priority is specified,
then a priority of 10 is given (this is a macro, and perhaps may be changed
in the future).

Now probes may be added to affect other probes that are attached to a
tracepoint with a guaranteed order.

One use case would be to allow tracing of tracepoints be able to filter by
pid. A special (higher priority probe) may be added to the sched_switch
tracepoint and set the necessary flags of the other tracepoints to notify
them if they should be traced or not. In case a tracepoint is enabled at the
sched_switch tracepoint too, the order of the two are not random.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index afada369c5b7..6b79537a42b1 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -26,6 +26,7 @@ struct notifier_block;
 struct tracepoint_func {
         void *func;
         void *data;
+        int prio;
 };
 
 struct tracepoint {
@@ -42,9 +43,14 @@ struct trace_enum_map {
         unsigned long           enum_value;
 };
 
+#define TRACEPOINT_DEFAULT_PRIO 10
+
 extern int
 tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
 extern int
+tracepoint_probe_register_prio(struct tracepoint *tp, void *probe, void *data,
+                               int prio);
+extern int
 tracepoint_probe_unregister(struct tracepoint *tp, void *probe, void *data);
 extern void
 for_each_kernel_tracepoint(void (*fct)(struct tracepoint *tp, void *priv),
@@ -207,6 +213,13 @@ extern void syscall_unregfunc(void);
                                                 (void *)probe, data);   \
         }                                                               \
         static inline int                                               \
+        register_trace_prio_##name(void (*probe)(data_proto), void *data,\
+                                   int prio)                            \
+        {                                                               \
+                return tracepoint_probe_register_prio(&__tracepoint_##name, \
+                                              (void *)probe, data, prio); \
+        }                                                               \
+        static inline int                                               \
         unregister_trace_##name(void (*probe)(data_proto), void *data)  \
         {                                                               \
                 return tracepoint_probe_unregister(&__tracepoint_##name,\