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

[Tony Lindgren]

* 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);