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 <[email protected]> Reported-by: Tony Lindgren <[email protected]> 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? 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);
