nfsd: wake blocked file lock waiters before sending callback

When a blocked NFS lock is "awoken" we send a callback to the server and
then wake any hosts waiting on it. If a client attempts to get a lock
and then drops off the net, we could end up waiting for a long time
until we end up waking locks blocked on that request.

So, wake any other waiting lock requests before sending the callback.
Do this by calling locks_delete_block in a new "prepare" phase for
CB_NOTIFY_LOCK callbacks.

URL: https://bugzilla.kernel.org/show_bug.cgi?id=203363
Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
Reported-by: Slawomir Pryczek <[e-mail]>
Cc: Neil Brown <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4state.c

···
 	}
 }
 
+static void
+nfsd4_cb_notify_lock_prepare(struct nfsd4_callback *cb)
+{
+	struct nfsd4_blocked_lock	*nbl = container_of(cb,
+						struct nfsd4_blocked_lock, nbl_cb);
+	locks_delete_block(&nbl->nbl_lock);
+}
+
 static int
 nfsd4_cb_notify_lock_done(struct nfsd4_callback *cb, struct rpc_task *task)
 {
···
 }
 
 static const struct nfsd4_callback_ops nfsd4_cb_notify_lock_ops = {
+	.prepare	= nfsd4_cb_notify_lock_prepare,
 	.done		= nfsd4_cb_notify_lock_done,
 	.release	= nfsd4_cb_notify_lock_release,
 };