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

From: Tony Lindgren
Date: Mon Feb 13 2017 - 12:57:44 EST


* Tejun Heo <tj@xxxxxxxxxx> [170211 12:34]:
> 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")
> ---
> Hello,
>
> Tony, can you please verify that this resolves the lockdep warnings
> that you've been seeing on linux-next?

Yes thanks this fixes the issue I was seeing:

Tested-by: Tony Lindgren <tony@xxxxxxxxxxx>

> 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
> top.
>
> Thanks!
>
> 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
> kn->attr.ops->release(of);
> 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);
> kfree(of->prealloc_buf);