if (!nfsd_file_mark_slab) {
                pr_err("nfsd: unable to create nfsd_file_mark_slab\n");
                goto out_err;
        }

Allocating that slabcache failed, so do_kmem_cache_create() called
__kmem_cache_release() on the way out. That triggered the lockdep
warning which is this in lockdep_unregister_key():

        WARN_ON_ONCE(!found && debug_locks);

So it tried to tear down a lockdep key, but it didn't exist?

This doesn't seem to be nfsd related either. I'll hand off to mm.

#syz set subsystems: mm

-- 
Jeff Layton <jlayton@kernel.org>[syzbot] [nfs?] [io-uring?] WARNING in nfsd_file_cache_initJeff Layton <jlayton@kernel.org> undefinedsyzbot <syzbot+a6f4d69b9b23404bbabf@syzkaller.appspotmail.com>, Dai.Ngo@oracle.com, axboe@kernel.dk, chuck.lever@oracle.com, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, neil@brown.name, okorniev@redhat.com, syzkaller-bugs@googlegroups.com, tom@talpey.com undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined‡Rƒ…@