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Commit f83f3c51 authored by Tejun Heo's avatar Tejun Heo
Browse files

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



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: default avatarTejun Heo <tj@kernel.org>
Reported-and-tested-by: default avatarTony Lindgren <tony@atomide.com>
Fixes: 0e67db2f ("kernfs: add kernfs_ops->open/release() callbacks")
Acked-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 63f1ca59

fs/kernfs/file.c

+14 −5
Original line number Diff line number Diff line
@@ -747,10 +747,15 @@ static int kernfs_fop_open(struct inode *inode, struct file *file) 
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 kernfs_node *kn, 
		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 inode *inode, struct file *filp) 
	struct kernfs_node *kn = filp->f_path.dentry->d_fsdata;

	struct kernfs_open_file *of = kernfs_of(filp);



	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);