tracepoint: Fix out of sync data passing by static caller

Naresh reported a bug that appears to be a side effect of the static
calls. It happens when going from more than one tracepoint callback to
a single one, and removing the first callback on the list. The list of
tracepoint callbacks holds data and a function to call with the
parameters of that tracepoint and a handler to the associated data.

 old_list:
	0: func = foo; data = NULL;
	1: func = bar; data = &bar_struct;

 new_list:
	0: func = bar; data = &bar_struct;

	CPU 0				CPU 1
	-----				-----
   tp_funcs = old_list;
   tp_static_caller = tp_interator

   __DO_TRACE()

    data = tp_funcs[0].data = NULL;

				   tp_funcs = new_list;
				   tracepoint_update_call()
				      tp_static_caller = tp_funcs[0] = bar;
    tp_static_caller(data)
       bar(data)
         x = data->item = NULL->item

       BOOM!

To solve this, add a tracepoint_synchronize_unregister() between
changing tp_funcs and updating the static tracepoint, that does both a
synchronize_rcu() and synchronize_srcu(). This will ensure that when
the static call is updated to the single callback that it will be
receiving the data that it registered with.

Fixes: d25e37d8 ("tracepoint: Optimize using static_call()")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/linux-next/CA+G9fYvPXVRO0NV7yL=FxCmFEMYkCwdz7R=9W+_votpT824YJA@mail.gmail.com

kernel/tracepoint.c

@@ -221,7 +221,7 @@ static void *func_remove(struct tracepoint_func **funcs,
 	return old;
 }
 
-static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func *tp_funcs)
+static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func *tp_funcs, bool sync)
 {
 	void *func = tp->iterator;
 
@@ -229,8 +229,17 @@ static void tracepoint_update_call(struct tracepoint *tp, struct tracepoint_func
 	if (!tp->static_call_key)
 		return;
 
-	if (!tp_funcs[1].func)
+	if (!tp_funcs[1].func) {
 		func = tp_funcs[0].func;
+		/*
+		 * If going from the iterator back to a single caller,
+		 * we need to synchronize with __DO_TRACE to make sure
+		 * that the data passed to the callback is the one that
+		 * belongs to that callback.
+		 */
+		if (sync)
+			tracepoint_synchronize_unregister();
+	}
 
 	__static_call_update(tp->static_call_key, tp->static_call_tramp, func);
 }
@@ -265,7 +274,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
 	 * include/linux/tracepoint.h using rcu_dereference_sched().
 	 */
 	rcu_assign_pointer(tp->funcs, tp_funcs);
-	tracepoint_update_call(tp, tp_funcs);
+	tracepoint_update_call(tp, tp_funcs, false);
 	static_key_enable(&tp->key);
 
 	release_probes(old);
@@ -297,11 +306,12 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 			tp->unregfunc();
 
 		static_key_disable(&tp->key);
+		rcu_assign_pointer(tp->funcs, tp_funcs);
 	} else {
-		tracepoint_update_call(tp, tp_funcs);
+		rcu_assign_pointer(tp->funcs, tp_funcs);
+		tracepoint_update_call(tp, tp_funcs,
+				       tp_funcs[0].func != old[0].func);
 	}
-
-	rcu_assign_pointer(tp->funcs, tp_funcs);
 	release_probes(old);
 	return 0;
 }