Subject: [PATCH v2 net 2/2] 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: 04c26faa51d1 ("tipc: wait and exit until all work queues are done")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: tipc-discussion@lists.sourceforge.net
---
net/tipc/core.c      | 4 ++--
net/tipc/udp_media.c | 4 +++-
2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/tipc/core.c b/net/tipc/core.c index

1ddecea1df6e9100334c47a28ff6c065292fb9ad..315975c3be8186784e9c44c9ff
69d62c17ffd4b9 100644

--- a/net/tipc/core.c
+++ b/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) diff --git
a/net/tipc/udp_media.c b/net/tipc/udp_media.c index
66f3cb87a0aaaac8f40e8f237ab9a44d539b1cd8..62ae7f5b58409c89798c915de
e752ac42487581f 100644

--- a/net/tipc/udp_media.c
+++ b/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);
}

--
2.55.0.rc0.799.gd6f94ed593-goog