tracing: Use kstrdup_const instead of private implementation

The kernel now has kstrdup_const/kfree_const for reusing .rodata
(typically string literals) when possible; there's no reason to
duplicate that logic in the tracing system. Moreover, as the comment
above core_kernel_data states, it may not always return true for
.rodata - that is for example the case on x86_64, where we thus end up
kstrdup'ing all the passed-in strings.

Arguably, testing for .rodata explicitly (as kstrdup_const does) is
also more correct: I don't think one is supposed to be able to change
the name after creating the event_subsystem by passing the address of
a static char (but non-const) array.

Link: http://lkml.kernel.org/r/1441833841-12955-1-git-send-email-linux@rasmusvillemoes.dk

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -38,21 +38,19 @@ static LIST_HEAD(ftrace_common_fields);
 static struct kmem_cache *field_cachep;
 static struct kmem_cache *file_cachep;
 
-#define SYSTEM_FL_FREE_NAME		(1 << 31)
-
 static inline int system_refcount(struct event_subsystem *system)
 {
-	return system->ref_count & ~SYSTEM_FL_FREE_NAME;
+	return system->ref_count;
 }
 
 static int system_refcount_inc(struct event_subsystem *system)
 {
-	return (system->ref_count++) & ~SYSTEM_FL_FREE_NAME;
+	return system->ref_count++;
 }
 
 static int system_refcount_dec(struct event_subsystem *system)
 {
-	return (--system->ref_count) & ~SYSTEM_FL_FREE_NAME;
+	return --system->ref_count;
 }
 
 /* Double loops, do not use break, only goto's work */
@@ -461,8 +459,7 @@ static void __put_system(struct event_subsystem *system)
 		kfree(filter->filter_string);
 		kfree(filter);
 	}
-	if (system->ref_count & SYSTEM_FL_FREE_NAME)
-		kfree(system->name);
+	kfree_const(system->name);
 	kfree(system);
 }
 
@@ -1493,13 +1490,9 @@ create_new_subsystem(const char *name)
 	system->ref_count = 1;
 
 	/* Only allocate if dynamic (kprobes and modules) */
-	if (!core_kernel_data((unsigned long)name)) {
-		system->ref_count |= SYSTEM_FL_FREE_NAME;
-		system->name = kstrdup(name, GFP_KERNEL);
-		if (!system->name)
-			goto out_free;
-	} else
-		system->name = name;
+	system->name = kstrdup_const(name, GFP_KERNEL);
+	if (!system->name)
+		goto out_free;
 
 	system->filter = NULL;
 
@@ -1512,8 +1505,7 @@ create_new_subsystem(const char *name)
 	return system;
 
  out_free:
-	if (system->ref_count & SYSTEM_FL_FREE_NAME)
-		kfree(system->name);
+	kfree_const(system->name);
 	kfree(system);
 	return NULL;
 }