tipc: fix lockdep warning during node delete

We see the following lockdep warning:

[ 2284.078521] ======================================================
[ 2284.078604] WARNING: possible circular locking dependency detected
[ 2284.078604] 4.19.0+ #42 Tainted: G            E
[ 2284.078604] ------------------------------------------------------
[ 2284.078604] rmmod/254 is trying to acquire lock:
[ 2284.078604] 00000000acd94e28 ((&n->timer)#2){+.-.}, at: del_timer_sync+0x5/0xa0
[ 2284.078604]
[ 2284.078604] but task is already holding lock:
[ 2284.078604] 00000000f997afc0 (&(&tn->node_list_lock)->rlock){+.-.}, at: tipc_node_stop+0xac/0x190 [tipc]
[ 2284.078604]
[ 2284.078604] which lock already depends on the new lock.
[ 2284.078604]
[ 2284.078604]
[ 2284.078604] the existing dependency chain (in reverse order) is:
[ 2284.078604]
[ 2284.078604] -> #1 (&(&tn->node_list_lock)->rlock){+.-.}:
[ 2284.078604]        tipc_node_timeout+0x20a/0x330 [tipc]
[ 2284.078604]        call_timer_fn+0xa1/0x280
[ 2284.078604]        run_timer_softirq+0x1f2/0x4d0
[ 2284.078604]        __do_softirq+0xfc/0x413
[ 2284.078604]        irq_exit+0xb5/0xc0
[ 2284.078604]        smp_apic_timer_interrupt+0xac/0x210
[ 2284.078604]        apic_timer_interrupt+0xf/0x20
[ 2284.078604]        default_idle+0x1c/0x140
[ 2284.078604]        do_idle+0x1bc/0x280
[ 2284.078604]        cpu_startup_entry+0x19/0x20
[ 2284.078604]        start_secondary+0x187/0x1c0
[ 2284.078604]        secondary_startup_64+0xa4/0xb0
[ 2284.078604]
[ 2284.078604] -> #0 ((&n->timer)#2){+.-.}:
[ 2284.078604]        del_timer_sync+0x34/0xa0
[ 2284.078604]        tipc_node_delete+0x1a/0x40 [tipc]
[ 2284.078604]        tipc_node_stop+0xcb/0x190 [tipc]
[ 2284.078604]        tipc_net_stop+0x154/0x170 [tipc]
[ 2284.078604]        tipc_exit_net+0x16/0x30 [tipc]
[ 2284.078604]        ops_exit_list.isra.8+0x36/0x70
[ 2284.078604]        unregister_pernet_operations+0x87/0xd0
[ 2284.078604]        unregister_pernet_subsys+0x1d/0x30
[ 2284.078604]        tipc_exit+0x11/0x6f2 [tipc]
[ 2284.078604]        __x64_sys_delete_module+0x1df/0x240
[ 2284.078604]        do_syscall_64+0x66/0x460
[ 2284.078604]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 2284.078604]
[ 2284.078604] other info that might help us debug this:
[ 2284.078604]
[ 2284.078604]  Possible unsafe locking scenario:
[ 2284.078604]
[ 2284.078604]        CPU0                    CPU1
[ 2284.078604]        ----                    ----
[ 2284.078604]   lock(&(&tn->node_list_lock)->rlock);
[ 2284.078604]                                lock((&n->timer)#2);
[ 2284.078604]                                lock(&(&tn->node_list_lock)->rlock);
[ 2284.078604]   lock((&n->timer)#2);
[ 2284.078604]
[ 2284.078604]  *** DEADLOCK ***
[ 2284.078604]
[ 2284.078604] 3 locks held by rmmod/254:
[ 2284.078604]  #0: 000000003368be9b (pernet_ops_rwsem){+.+.}, at: unregister_pernet_subsys+0x15/0x30
[ 2284.078604]  #1: 0000000046ed9c86 (rtnl_mutex){+.+.}, at: tipc_net_stop+0x144/0x170 [tipc]
[ 2284.078604]  #2: 00000000f997afc0 (&(&tn->node_list_lock)->rlock){+.-.}, at: tipc_node_stop+0xac/0x19
[...}

The reason is that the node timer handler sometimes needs to delete a
node which has been disconnected for too long. To do this, it grabs
the lock 'node_list_lock', which may at the same time be held by the
generic node cleanup function, tipc_node_stop(), during module removal.
Since the latter is calling del_timer_sync() inside the same lock, we
have a potential deadlock.

We fix this letting the timer cleanup function use spin_trylock()
instead of just spin_lock(), and when it fails to grab the lock it
just returns so that the timer handler can terminate its execution.
This is safe to do, since tipc_node_stop() anyway is about to
delete both the timer and the node instance.

Fixes: 6a939f365bdb ("tipc: Auto removal of peer down node instance")
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/node.c b/net/tipc/node.c
index 2afc4f8..4880197 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -584,12 +584,15 @@
 /* tipc_node_cleanup - delete nodes that does not
  * have active links for NODE_CLEANUP_AFTER time
  */
-static int tipc_node_cleanup(struct tipc_node *peer)
+static bool tipc_node_cleanup(struct tipc_node *peer)
 {
 	struct tipc_net *tn = tipc_net(peer->net);
 	bool deleted = false;
 
-	spin_lock_bh(&tn->node_list_lock);
+	/* If lock held by tipc_node_stop() the node will be deleted anyway */
+	if (!spin_trylock_bh(&tn->node_list_lock))
+		return false;
+
 	tipc_node_write_lock(peer);
 
 	if (!node_is_up(peer) && time_after(jiffies, peer->delete_at)) {