tracing/events: fix concurrent access to ftrace_events list

A module will add/remove its trace events when it gets loaded/unloaded, so
the ftrace_events list is not "const", and concurrent access needs to be
protected.

This patch thus fixes races between loading/unloding modules and read
'available_events' or read/write 'set_event', etc.

Below shows how to reproduce the race:

 # for ((; ;)) { cat /mnt/tracing/available_events; } > /dev/null &
 # for ((; ;)) { insmod trace-events-sample.ko; rmmod sample; } &

After a while:

BUG: unable to handle kernel paging request at 0010011c
IP: [<c1080f27>] t_next+0x1b/0x2d
...
Call Trace:
 [<c10c90e6>] ? seq_read+0x217/0x30d
 [<c10c8ecf>] ? seq_read+0x0/0x30d
 [<c10b4c19>] ? vfs_read+0x8f/0x136
 [<c10b4fc3>] ? sys_read+0x40/0x65
 [<c1002a68>] ? sysenter_do_call+0x12/0x36

[ Impact: fix races when concurrent accessing ftrace_events list ]

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <4A00F709.3080800@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 14 insertions, 5 deletions

diff --git a/kernel/trace/trace_event_profile.c b/kernel/trace/trace_event_profile.c
index 7bf2ad65eee5..5b5895afecfe 100644
--- a/kernel/trace/trace_event_profile.c
+++ b/kernel/trace/trace_event_profile.c
@@ -10,21 +10,30 @@
 int ftrace_profile_enable(int event_id)
 {
         struct ftrace_event_call *event;
+        int ret = -EINVAL;
 
+        mutex_lock(&event_mutex);
         list_for_each_entry(event, &ftrace_events, list) {
-                if (event->id == event_id)
-                        return event->profile_enable(event);
+                if (event->id == event_id) {
+                        ret = event->profile_enable(event);
+                        break;
+                }
         }
+        mutex_unlock(&event_mutex);
 
-        return -EINVAL;
+        return ret;
 }
 
 void ftrace_profile_disable(int event_id)
 {
         struct ftrace_event_call *event;
 
+        mutex_lock(&event_mutex);
         list_for_each_entry(event, &ftrace_events, list) {
-                if (event->id == event_id)
-                        return event->profile_disable(event);
+                if (event->id == event_id) {
+                        event->profile_disable(event);
+                        break;
+                }
         }
+        mutex_unlock(&event_mutex);
 }