Patch "tipc: fix nametbl_lock soft lockup at node/link events" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

tipc: fix nametbl_lock soft lockup at node/link events

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tipc-fix-nametbl_lock-soft-lockup-at-node-link-events.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Parthasarathy Bhuvaragan <[email protected]>
Date: Tue, 24 Jan 2017 13:00:43 +0100
Subject: tipc: fix nametbl_lock soft lockup at node/link events

From: Parthasarathy Bhuvaragan <[email protected]>

[ Upstream commit 93f955aad4bacee5acebad141d1a03cd51f27b4e ]

We trigger a soft lockup as we grab nametbl_lock twice if the node has a pending node up/down or link up/down event while:
- we process an incoming named message in tipc_named_rcv() and perform an tipc_update_nametbl().
- we have pending backlog items in the name distributor queue during a nametable update using tipc_nametbl_publish() or tipc_nametbl_withdraw().

The following are the call chain associated:
tipc_named_rcv() Grabs nametbl_lock
tipc_update_nametbl() (publish/withdraw)
tipc_node_subscribe()/unsubscribe()
tipc_node_write_unlock()
<< lockup occurs if an outstanding node/link event
exits, as we grabs nametbl_lock again >>

tipc_nametbl_withdraw() Grab nametbl_lock
tipc_named_process_backlog()
tipc_update_nametbl()
<< rest as above >>

The function tipc_node_write_unlock(), in addition to releasing the lock processes the outstanding node/link up/down events. To do this, we need to grab the nametbl_lock again leading to the lockup.

In this commit we fix the soft lockup by introducing a fast variant of node_unlock(), where we just release the lock. We adapt the node_subscribe()/node_unsubscribe() to use the fast variants.

---
 net/tipc/node.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -263,6 +263,11 @@ static void tipc_node_write_lock(struct
 	write_lock_bh(&n->lock);
 }
 
+static void tipc_node_write_unlock_fast(struct tipc_node *n)
+{
+	write_unlock_bh(&n->lock);
+}
+
 static void tipc_node_write_unlock(struct tipc_node *n)
 {
 	struct net *net = n->net;

@@ -417,7 +422,7 @@ void tipc_node_subscribe(struct net *net
 	}
 	tipc_node_write_lock(n);
 	list_add_tail(subscr, &n->publ_list);
-	tipc_node_write_unlock(n);
+	tipc_node_write_unlock_fast(n);
 	tipc_node_put(n);
 }
 

@@ -435,7 +440,7 @@ void tipc_node_unsubscribe(struct net *n
 	}
 	tipc_node_write_lock(n);
 	list_del_init(subscr);
-	tipc_node_write_unlock(n);
+	tipc_node_write_unlock_fast(n);
 	tipc_node_put(n);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/tipc-fix-connection-refcount-error.patch queue-4.9/tipc-fix-nametbl_lock-soft-lockup-at-node-link-events.patch queue-4.9/tipc-ignore-requests-when-the-connection-state-is-not-connected.patch queue-4.9/tipc-add-subscription-refcount-to-avoid-invalid-delete.patch