tipc: introduce new spinlock to protect struct link_req

Currently, only 'bearer_lock' is used to protect struct link_req in
the function disc_timeout(). This is unsafe, since the member fields
'num_nodes' and 'timer_intv' might be accessed by below three different
threads simultaneously, none of them grabbing bearer_lock in the
critical region:

link_activate()
  tipc_bearer_add_dest()
    tipc_disc_add_dest()
      req->num_nodes++;

tipc_link_reset()
  tipc_bearer_remove_dest()
    tipc_disc_remove_dest()
      req->num_nodes--
      disc_update()
        read req->num_nodes
	write req->timer_intv

disc_timeout()
  read req->num_nodes
  read/write req->timer_intv

Without lock protection, the only symptom of a race is that discovery
messages occasionally may not be sent out. This is not fatal, since such
messages are best-effort anyway. On the other hand, since discovery
messages are not time critical, adding a protecting lock brings no
serious overhead either. So we add a new, dedicated spinlock in
order to guarantee absolute data consistency in link_req objects.
This also helps reduce the overall role of the bearer_lock, which
we want to remove completely in a later commit series.
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/discover.c

@@ -50,6 +50,7 @@
  * @dest: destination address for request messages
  * @domain: network domain to which links can be established
  * @num_nodes: number of nodes currently discovered (i.e. with an active link)
+ * @lock: spinlock for controlling access to requests
  * @buf: request message to be (repeatedly) sent
  * @timer: timer governing period between requests
  * @timer_intv: current interval between requests (in ms)
@@ -59,6 +60,7 @@ struct tipc_link_req {
 	struct tipc_media_addr dest;
 	u32 domain;
 	int num_nodes;
+	spinlock_t lock;
 	struct sk_buff *buf;
 	struct timer_list timer;
 	unsigned int timer_intv;
@@ -274,7 +276,9 @@ static void disc_update(struct tipc_link_req *req)
  */
 void tipc_disc_add_dest(struct tipc_link_req *req)
 {
+	spin_lock_bh(&req->lock);
 	req->num_nodes++;
+	spin_unlock_bh(&req->lock);
 }
 
 /**
@@ -283,8 +287,10 @@ void tipc_disc_add_dest(struct tipc_link_req *req)
  */
 void tipc_disc_remove_dest(struct tipc_link_req *req)
 {
+	spin_lock_bh(&req->lock);
 	req->num_nodes--;
 	disc_update(req);
+	spin_unlock_bh(&req->lock);
 }
 
 /**
@@ -297,7 +303,7 @@ static void disc_timeout(struct tipc_link_req *req)
 {
 	int max_delay;
 
-	spin_lock_bh(&req->bearer->lock);
+	spin_lock_bh(&req->lock);
 
 	/* Stop searching if only desired node has been found */
 	if (tipc_node(req->domain) && req->num_nodes) {
@@ -325,7 +331,7 @@ static void disc_timeout(struct tipc_link_req *req)
 
 	k_start_timer(&req->timer, req->timer_intv);
 exit:
-	spin_unlock_bh(&req->bearer->lock);
+	spin_unlock_bh(&req->lock);
 }
 
 /**
@@ -356,6 +362,7 @@ int tipc_disc_create(struct tipc_bearer *b_ptr, struct tipc_media_addr *dest,
 	req->domain = dest_domain;
 	req->num_nodes = 0;
 	req->timer_intv = TIPC_LINK_REQ_INIT;
+	spin_lock_init(&req->lock);
 	k_init_timer(&req->timer, (Handler)disc_timeout, (unsigned long)req);
 	k_start_timer(&req->timer, req->timer_intv);
 	b_ptr->link_req = req;