[PATCH] kernfs: fix locking around kernfs_ops->release() callback

From: Tejun Heo
Date: Sat Feb 11 2017 - 15:33:11 EST

The release callback may be called from two places - file release
operation and kernfs open file draining. kernfs_open_file->mutex is
used to synchronize the two callsites. This unfortunately leads to
possible circular locking because of->mutex is used to protect the
usual kernfs operations which may use locking constructs which are
held while removing and thus draining kernfs files.

@of->mutex is for synchronizing concurrent kernfs access operations
and all we need here is synchronization between the releaes and drain
paths. As the drain path has to grab kernfs_open_file_mutex anyway,
let's use the mutex to synchronize the release operation instead.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Tony Lindgren <tony@xxxxxxxxxxx>
Fixes: 0e67db2f9fe9 ("kernfs: add kernfs_ops->open/release() callbacks")

Tony, can you please verify that this resolves the lockdep warnings
that you've been seeing on linux-next?

Greg, this is a fix for the kernfs patches which are being routed
through the cgroup tree. Once Tony confirms, I'll apply this patch on


fs/kernfs/file.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -747,10 +747,15 @@ err_out:
static void kernfs_release_file(struct kernfs_node *kn,
struct kernfs_open_file *of)
- if (!(kn->flags & KERNFS_HAS_RELEASE))
- return;
+ /*
+ * @of is guaranteed to have no other file operations in flight and
+ * we just want to synchronize release and drain paths.
+ * @kernfs_open_file_mutex is enough. @of->mutex can't be used
+ * here because drain path may be called from places which can
+ * cause circular dependency.
+ */
+ lockdep_assert_held(&kernfs_open_file_mutex);

- mutex_lock(&of->mutex);
if (!of->released) {
* A file is never detached without being released and we
@@ -760,7 +765,6 @@ static void kernfs_release_file(struct k
of->released = true;
- mutex_unlock(&of->mutex);

static int kernfs_fop_release(struct inode *inode, struct file *filp)
@@ -768,7 +772,12 @@ static int kernfs_fop_release(struct ino
struct kernfs_node *kn = filp->f_path.dentry->d_fsdata;
struct kernfs_open_file *of = kernfs_of(filp);

- kernfs_release_file(kn, of);
+ if (kn->flags & KERNFS_HAS_RELEASE) {
+ mutex_lock(&kernfs_open_file_mutex);
+ kernfs_release_file(kn, of);
+ mutex_unlock(&kernfs_open_file_mutex);
+ }
kernfs_put_open_node(kn, of);
seq_release(inode, filp);