tracing/kprobes: Kill probe_enable_lock

enable_trace_probe() and disable_trace_probe() should not worry about
serialization, the caller (perf_trace_init or __ftrace_set_clr_event)
holds event_mutex.

They are also called by kprobe_trace_self_tests_init(), but this __init
function can't race with itself or trace_events.c

And note that this code depended on event_mutex even before 41a7dd420c
which introduced probe_enable_lock. In fact it assumes that the caller
kprobe_register() can never race with itself. Otherwise, say, tp->flags
manipulations are racy.

Link: http://lkml.kernel.org/r/20130620173809.GA13158@redhat.com

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 20 insertions, 23 deletions

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index c35bebe53ffe..282f86cfd304 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -183,16 +183,15 @@ static struct trace_probe *find_trace_probe(const char *event,
         return NULL;
 }
 
+/*
+ * This and enable_trace_probe/disable_trace_probe rely on event_mutex
+ * held by the caller, __ftrace_set_clr_event().
+ */
 static int trace_probe_nr_files(struct trace_probe *tp)
 {
-        struct ftrace_event_file **file;
+        struct ftrace_event_file **file = rcu_dereference_raw(tp->files);
         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++;
@@ -200,8 +199,6 @@ static int trace_probe_nr_files(struct trace_probe *tp)
         return ret;
 }
 
-static DEFINE_MUTEX(probe_enable_lock);
-
 /*
  * Enable trace_probe
  * if the file is NULL, enable "perf" handler, or enable "trace" handler.
@@ -211,8 +208,6 @@ enable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
 {
         int ret = 0;
 
-        mutex_lock(&probe_enable_lock);
-
         if (file) {
                 struct ftrace_event_file **new, **old;
                 int n = trace_probe_nr_files(tp);
@@ -223,7 +218,7 @@ enable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
                               GFP_KERNEL);
                 if (!new) {
                         ret = -ENOMEM;
-                        goto out_unlock;
+                        goto out;
                 }
                 memcpy(new, old, n * sizeof(struct ftrace_event_file *));
                 new[n] = file;
@@ -246,10 +241,7 @@ enable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
                 else
                         ret = enable_kprobe(&tp->rp.kp);
         }
-
- out_unlock:
-        mutex_unlock(&probe_enable_lock);
-
+ out:
         return ret;
 }
 
@@ -282,8 +274,6 @@ disable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
 {
         int ret = 0;
 
-        mutex_lock(&probe_enable_lock);
-
         if (file) {
                 struct ftrace_event_file **new, **old;
                 int n = trace_probe_nr_files(tp);
@@ -292,7 +282,7 @@ disable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
                 old = rcu_dereference_raw(tp->files);
                 if (n == 0 || trace_probe_file_index(tp, file) < 0) {
                         ret = -EINVAL;
-                        goto out_unlock;
+                        goto out;
                 }
 
                 if (n == 1) {   /* Remove the last file */
@@ -303,7 +293,7 @@ disable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
                                       GFP_KERNEL);
                         if (!new) {
                                 ret = -ENOMEM;
-                                goto out_unlock;
+                                goto out;
                         }
 
                         /* This copy & check loop copies the NULL stopper too */
@@ -326,10 +316,7 @@ disable_trace_probe(struct trace_probe *tp, struct ftrace_event_file *file)
                 else
                         disable_kprobe(&tp->rp.kp);
         }
-
- out_unlock:
-        mutex_unlock(&probe_enable_lock);
-
+ out:
         return ret;
 }
 
@@ -1214,6 +1201,12 @@ kretprobe_perf_func(struct trace_probe *tp, struct kretprobe_instance *ri,
 }
 #endif  /* CONFIG_PERF_EVENTS */
 
+/*
+ * called by perf_trace_init() or __ftrace_set_clr_event() under event_mutex.
+ *
+ * kprobe_trace_self_tests_init() does enable_trace_probe/disable_trace_probe
+ * lockless, but we can't race with this __init function.
+ */
 static __kprobes
 int kprobe_register(struct ftrace_event_call *event,
                     enum trace_reg type, void *data)
@@ -1379,6 +1372,10 @@ find_trace_probe_file(struct trace_probe *tp, struct trace_array *tr)
         return NULL;
 }
 
+/*
+ * Nobody but us can call enable_trace_probe/disable_trace_probe at this
+ * stage, we can do this lockless.
+ */
 static __init int kprobe_trace_self_tests_init(void)
 {
         int ret, warn = 0;