[PATCH 2/3] tracing/kprobes: Kill probe_enable_lock

[Oleg Nesterov <oleg@redhat.com>]

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/trace/trace_kprobe.c |   33 ++++++++++-----------------------
 1 files changed, 10 insertions(+), 23 deletions(-)

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index c0af476..5a73de0 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;
@@ -247,10 +242,7 @@ 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;
 }
 
@@ -283,8 +275,6 @@ 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);
@@ -293,7 +283,7 @@ 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 */
@@ -304,7 +294,7 @@ 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 */
@@ -327,10 +317,7 @@ 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;
 }
 
-- 
1.5.5.1