blktrace: fix a race when creating blk_tree_root in debugfs

t1                                t2
------                            ------
do_blk_trace_setup()              do_blk_trace_setup()
  if (!blk_tree_root) {
                                    if (!blk_tree_root)
    blk_tree_root = create_dir()
                                      blk_tree_root = create_dir();
                                      (now blk_tree_root == NULL)
  ...
  dir = create_dir(name, blk_tree_root);

Due to this race, t1 will create 'dir' in /debugfs but not /debugfs/block.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index ee7a8bb8b1e..95f89faca73 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -426,11 +426,15 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 
         ret = -ENOENT;
 
+        mutex_lock(&blk_tree_mutex);
         if (!blk_tree_root) {
                 blk_tree_root = debugfs_create_dir("block", NULL);
-                if (!blk_tree_root)
+                if (!blk_tree_root) {
+                        mutex_unlock(&blk_tree_mutex);
                         goto err;
+                }
         }
+        mutex_unlock(&blk_tree_mutex);
 
         dir = debugfs_create_dir(buts->name, blk_tree_root);