[tip:tracing/blktrace] blktrace: fix a race when creating blk_tree_root in debugfs

[Li Zefan]

Commit-ID:  a216c4acfafb28d697fe07b4137ab780131e4d60
Gitweb:     http://git.kernel.org/tip/a216c4acfafb28d697fe07b4137ab780131e4d60
Author:     Li Zefan <lizf@xxxxxxxxxxxxxx>
AuthorDate: Wed, 25 Mar 2009 17:19:33 +0800
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Wed, 25 Mar 2009 12:05:39 +0100

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@xxxxxxxxxxxxxx>
Acked-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
LKML-Reference: <49C9F725.9080409@xxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


---
 kernel/trace/blktrace.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index ee7a8bb..95f89fa 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);
 
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