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>

2 files changed, 63 insertions, 11 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,\
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 3490407dc7b7..ecd536de603a 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -91,11 +91,13 @@ static void debug_print_probes(struct tracepoint_func *funcs)
                 printk(KERN_DEBUG "Probe %d : %p\n", i, funcs[i].func);
 }
 
-static struct tracepoint_func *func_add(struct tracepoint_func **funcs,
-                struct tracepoint_func *tp_func)
+static struct tracepoint_func *
+func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
+         int prio)
 {
-        int nr_probes = 0;
         struct tracepoint_func *old, *new;
+        int nr_probes = 0;
+        int pos = -1;
 
         if (WARN_ON(!tp_func->func))
                 return ERR_PTR(-EINVAL);
@@ -104,18 +106,33 @@ static struct tracepoint_func *func_add(struct tracepoint_func **funcs,
         old = *funcs;
         if (old) {
                 /* (N -> N+1), (N != 0, 1) probes */
-                for (nr_probes = 0; old[nr_probes].func; nr_probes++)
+                for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
+                        /* Insert before probes of lower priority */
+                        if (pos < 0 && old[nr_probes].prio < prio)
+                                pos = nr_probes;
                         if (old[nr_probes].func == tp_func->func &&
                             old[nr_probes].data == tp_func->data)
                                 return ERR_PTR(-EEXIST);
+                }
         }
         /* + 2 : one for new probe, one for NULL func */
         new = allocate_probes(nr_probes + 2);
         if (new == NULL)
                 return ERR_PTR(-ENOMEM);
-        if (old)
-                memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
-        new[nr_probes] = *tp_func;
+        if (old) {
+                if (pos < 0) {
+                        pos = nr_probes;
+                        memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
+                } else {
+                        /* Copy higher priority probes ahead of the new probe */
+                        memcpy(new, old, pos * sizeof(struct tracepoint_func));
+                        /* Copy the rest after it. */
+                        memcpy(new + pos + 1, old + pos,
+                               (nr_probes - pos) * sizeof(struct tracepoint_func));
+                }
+        } else
+                pos = 0;
+        new[pos] = *tp_func;
         new[nr_probes + 1].func = NULL;
         *funcs = new;
         debug_print_probes(*funcs);
@@ -174,7 +191,7 @@ static void *func_remove(struct tracepoint_func **funcs,
  * Add the probe function to a tracepoint.
  */
 static int tracepoint_add_func(struct tracepoint *tp,
-                struct tracepoint_func *func)
+                               struct tracepoint_func *func, int prio)
 {
         struct tracepoint_func *old, *tp_funcs;
 
@@ -183,7 +200,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
 
         tp_funcs = rcu_dereference_protected(tp->funcs,
                         lockdep_is_held(&tracepoints_mutex));
-        old = func_add(&tp_funcs, func);
+        old = func_add(&tp_funcs, func, prio);
         if (IS_ERR(old)) {
                 WARN_ON_ONCE(1);
                 return PTR_ERR(old);
@@ -240,6 +257,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
  * @tp: tracepoint
  * @probe: probe handler
  * @data: tracepoint data
+ * @prio: priority of this function over other registered functions
  *
  * Returns 0 if ok, error value on error.
  * Note: if @tp is within a module, the caller is responsible for
@@ -247,7 +265,8 @@ static int tracepoint_remove_func(struct tracepoint *tp,
  * performed either with a tracepoint module going notifier, or from
  * within module exit functions.
  */
-int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
+int tracepoint_probe_register_prio(struct tracepoint *tp, void *probe,
+                                   void *data, int prio)
 {
         struct tracepoint_func tp_func;
         int ret;
@@ -255,10 +274,30 @@ int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
         mutex_lock(&tracepoints_mutex);
         tp_func.func = probe;
         tp_func.data = data;
-        ret = tracepoint_add_func(tp, &tp_func);
+        tp_func.prio = prio;
+        ret = tracepoint_add_func(tp, &tp_func, prio);
         mutex_unlock(&tracepoints_mutex);
         return ret;
 }
+EXPORT_SYMBOL_GPL(tracepoint_probe_register_prio);
+
+/**
+ * tracepoint_probe_register -  Connect a probe to a tracepoint
+ * @tp: tracepoint
+ * @probe: probe handler
+ * @data: tracepoint data
+ * @prio: priority of this function over other registered functions
+ *
+ * Returns 0 if ok, error value on error.
+ * Note: if @tp is within a module, the caller is responsible for
+ * unregistering the probe before the module is gone. This can be
+ * performed either with a tracepoint module going notifier, or from
+ * within module exit functions.
+ */
+int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
+{
+        return tracepoint_probe_register_prio(tp, probe, data, TRACEPOINT_DEFAULT_PRIO);
+}
 EXPORT_SYMBOL_GPL(tracepoint_probe_register);
 
 /**