net: give more chances to rcu in netdev_wait_allrefs_any()

This came while reviewing commit c4e86b4363 ("net: add two more
call_rcu_hurry()").

Paolo asked if adding one synchronize_rcu() would help.

While synchronize_rcu() does not help, making sure to call
rcu_barrier() before msleep(wait) is definitely helping
to make sure lazy call_rcu() are completed.

Instead of waiting ~100 seconds in my tests, the ref_tracker
splats occurs one time only, and netdev_wait_allrefs_any()
latency is reduced to the strict minimum.

Ideally we should audit our call_rcu() users to make sure
no refcount (or cascading call_rcu()) is held too long,
because rcu_barrier() is quite expensive.

Fixes: 0e4be9e57e ("net: use exponential backoff in netdev_wait_allrefs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/all/28bbf698-befb-42f6-b561-851c67f464aa@kernel.org/T/#m76d73ed6b03cd930778ac4d20a777f22a08d6824
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -10566,8 +10566,9 @@ static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 			rebroadcast_time = jiffies;
 		}
 
+		rcu_barrier();
+
 		if (!wait) {
-			rcu_barrier();
 			wait = WAIT_REFS_MIN_MSECS;
 		} else {
 			msleep(wait);