tipc: avoid busy looping in tipc_exit_net()

Blamed commit introduced a busy-wait loop in tipc_exit_net()
to wait for pending UDP bearer cleanup works to complete:

       while (atomic_read(&tn->wq_count))
               cond_resched();

This loop can busy-wait for a long time if cond_resched() is a NOP. This
typically happens if the netns exit is executed by a high priority task,
or under kernels configured without preemption (CONFIG_PREEMPT_NONE). In
such cases, it wastes CPU cycles and can lead to soft lockups.

Fix this by replacing the busy loop with wait_var_event(), allowing the
thread to sleep properly until the work queue count reaches zero.

Accordingly, update cleanup_bearer() to use atomic_dec_and_test() and
wake_up_var() to wake up the waiter when the count drops to zero.

This uses the global wait queue hash table, avoiding the need to bloat
struct tipc_net with a wait_queue_head_t. The atomic_dec_and_test()
provides the necessary memory barrier to ensure the wakeup is not missed.

Fixes: 04c26faa51 ("tipc: wait and exit until all work queues are done")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: tipc-discussion@lists.sourceforge.net
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260623173030.2925059-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/core.c

@@ -45,6 +45,7 @@
 #include "crypto.h"
 
 #include <linux/module.h>
+#include <linux/wait_bit.h>
 
 /* configurable TIPC parameters */
 unsigned int tipc_net_id __read_mostly;
@@ -118,8 +119,7 @@ static void __net_exit tipc_exit_net(struct net *net)
 #ifdef CONFIG_TIPC_CRYPTO
 	tipc_crypto_stop(&tipc_net(net)->crypto_tx);
 #endif
-	while (atomic_read(&tn->wq_count))
-		cond_resched();
+	wait_var_event(&tn->wq_count, atomic_read(&tn->wq_count) == 0);
 }
 
 static void __net_exit tipc_pernet_pre_exit(struct net *net)

net/tipc/udp_media.c

@@ -40,6 +40,7 @@
 #include <linux/igmp.h>
 #include <linux/kernel.h>
 #include <linux/workqueue.h>
+#include <linux/wait_bit.h>
 #include <linux/list.h>
 #include <net/sock.h>
 #include <net/ip.h>
@@ -830,7 +831,8 @@ static void cleanup_bearer(struct work_struct *work)
 	synchronize_net();
 
 	dst_cache_destroy(&ub->rcast.dst_cache);
-	atomic_dec(&tn->wq_count);
+	if (atomic_dec_and_test(&tn->wq_count))
+		wake_up_var(&tn->wq_count);
 	kfree(ub);
 }