nfsd: Don't release the callback slot unless it was actually held

BugLink: https://bugs.launchpad.net/bugs/1830176

commit e6abc8ca upstream.

If there are multiple callbacks queued, waiting for the callback
slot when the callback gets shut down, then they all currently
end up acting as if they hold the slot, and call
nfsd4_cb_sequence_done() resulting in interesting side-effects.

In addition, the 'retry_nowait' path in nfsd4_cb_sequence_done()
causes a loop back to nfsd4_cb_prepare() without first freeing the
slot, which causes a deadlock when nfsd41_cb_get_slot() gets called
a second time.

This patch therefore adds a boolean to track whether or not the
callback did pick up the slot, so that it can do the right thing
in these 2 cases.

Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/nfsd/nfs4callback.c

@@ -874,8 +874,9 @@ static void nfsd4_cb_prepare(struct rpc_task *task, void *calldata)
 	cb->cb_seq_status = 1;
 	cb->cb_status = 0;
 	if (minorversion) {
-		if (!nfsd41_cb_get_slot(clp, task))
+		if (!cb->cb_holds_slot && !nfsd41_cb_get_slot(clp, task))
 			return;
+		cb->cb_holds_slot = true;
 	}
 	rpc_call_start(task);
 }
@@ -902,6 +903,9 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
 		return true;
 	}
 
+	if (!cb->cb_holds_slot)
+		goto need_restart;
+
 	switch (cb->cb_seq_status) {
 	case 0:
 		/*
@@ -939,6 +943,7 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
 			cb->cb_seq_status);
 	}
 
+	cb->cb_holds_slot = false;
 	clear_bit(0, &clp->cl_cb_slot_busy);
 	rpc_wake_up_next(&clp->cl_cb_waitq);
 	dprintk("%s: freed slot, new seqid=%d\n", __func__,
@@ -1146,6 +1151,7 @@ void nfsd4_init_cb(struct nfsd4_callback *cb, struct nfs4_client *clp,
 	cb->cb_seq_status = 1;
 	cb->cb_status = 0;
 	cb->cb_need_restart = false;
+	cb->cb_holds_slot = false;
 }
 
 void nfsd4_run_cb(struct nfsd4_callback *cb)

fs/nfsd/state.h

@@ -70,6 +70,7 @@ struct nfsd4_callback {
 	int cb_seq_status;
 	int cb_status;
 	bool cb_need_restart;
+	bool cb_holds_slot;
 };
 
 struct nfsd4_callback_ops {