kernfs: fix locking around kernfs_ops->release() callback (f83f3c51) · Commits · 戴 / test

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@kernel.org> Reported-and-tested-by:

Tony Lindgren <tony@atomide.com> Fixes: ("kernfs: add kernfs_ops->open/release() callbacks") Acked-by:

Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/file.c

+14 −5

Original line number	Diff line number	Diff line
		@@ -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 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);

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