tracing/kprobes: Use rcu_dereference_raw for tp->files

Use rcu_dereference_raw() for accessing tp->files. Because the
write-side uses rcu_assign_pointer() for memory barrier,
the read-side also has to use rcu_dereference_raw() with
read memory barrier.

Link: http://lkml.kernel.org/r/20130513115834.6545.17022.stgit@mhiramat-M0-7522

Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tom Zanussi <tom.zanussi@intel.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_kprobe.c

@@ -185,9 +185,14 @@ static struct trace_probe *find_trace_probe(const char *event,
 
 static int trace_probe_nr_files(struct trace_probe *tp)
 {
-	struct ftrace_event_file **file = tp->files;
+	struct ftrace_event_file **file;
 	int ret = 0;
 
+	/*
+	 * Since all tp->files updater is protected by probe_enable_lock,
+	 * we don't need to lock an rcu_read_lock.
+	 */
+	file = rcu_dereference_raw(tp->files);
 	if (file)
 		while (*(file++))
 			ret++;
@@ -209,9 +214,10 @@ enable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
 	mutex_lock(&probe_enable_lock);
 
 	if (file) {
-		struct ftrace_event_file **new, **old = tp->files;
+		struct ftrace_event_file **new, **old;
 		int n = trace_probe_nr_files(tp);
 
+		old = rcu_dereference_raw(tp->files);
 		/* 1 is for new one and 1 is for stopper */
 		new = kzalloc((n + 2) * sizeof(struct ftrace_event_file *),
 			      GFP_KERNEL);
@@ -251,11 +257,17 @@ enable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
 static int
 trace_probe_file_index(struct trace_probe *tp, struct ftrace_event_file *file)
 {
+	struct ftrace_event_file **files;
 	int i;
 
-	if (tp->files) {
-		for (i = 0; tp->files[i]; i++)
-			if (tp->files[i] == file)
+	/*
+	 * Since all tp->files updater is protected by probe_enable_lock,
+	 * we don't need to lock an rcu_read_lock.
+	 */
+	files = rcu_dereference_raw(tp->files);
+	if (files) {
+		for (i = 0; files[i]; i++)
+			if (files[i] == file)
 				return i;
 	}
 
@@ -274,10 +286,11 @@ disable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
 	mutex_lock(&probe_enable_lock);
 
 	if (file) {
-		struct ftrace_event_file **new, **old = tp->files;
+		struct ftrace_event_file **new, **old;
 		int n = trace_probe_nr_files(tp);
 		int i, j;
 
+		old = rcu_dereference_raw(tp->files);
 		if (n == 0 || trace_probe_file_index(tp, file) < 0) {
 			ret = -EINVAL;
 			goto out_unlock;
@@ -872,9 +885,16 @@ __kprobe_trace_func(struct trace_probe *tp, struct pt_regs *regs,
 static __kprobes void
 kprobe_trace_func(struct trace_probe *tp, struct pt_regs *regs)
 {
-	struct ftrace_event_file **file = tp->files;
+	/*
+	 * Note: preempt is already disabled around the kprobe handler.
+	 * However, we still need an smp_read_barrier_depends() corresponding
+	 * to smp_wmb() in rcu_assign_pointer() to access the pointer.
+	 */
+	struct ftrace_event_file **file = rcu_dereference_raw(tp->files);
+
+	if (unlikely(!file))
+		return;
 
-	/* Note: preempt is already disabled around the kprobe handler */
 	while (*file) {
 		__kprobe_trace_func(tp, regs, *file);
 		file++;
@@ -925,9 +945,16 @@ static __kprobes void
 kretprobe_trace_func(struct trace_probe *tp, struct kretprobe_instance *ri,
 		     struct pt_regs *regs)
 {
-	struct ftrace_event_file **file = tp->files;
+	/*
+	 * Note: preempt is already disabled around the kprobe handler.
+	 * However, we still need an smp_read_barrier_depends() corresponding
+	 * to smp_wmb() in rcu_assign_pointer() to access the pointer.
+	 */
+	struct ftrace_event_file **file = rcu_dereference_raw(tp->files);
+
+	if (unlikely(!file))
+		return;
 
-	/* Note: preempt is already disabled around the kprobe handler */
 	while (*file) {
 		__kretprobe_trace_func(tp, ri, regs, *file);
 		file++;