net: use exponential backoff in netdev_wait_allrefs

The combination of aca_free_rcu, introduced in commit 2384d02520
("net/ipv6: Add anycast addresses to a global hashtable"), and
fib6_info_destroy_rcu, introduced in commit 9b0a8da8c4 ("net/ipv6:
respect rcu grace period before freeing fib6_info"), can result in
an extra rcu grace period being needed when deleting an interface,
with the result that netdev_wait_allrefs ends up hitting the msleep(250),
which is considerably longer than the required grace period.
This can result in long delays when deleting a large number of interfaces,
and it can be observed with this script:

ns=dummy-ns
NIFS=100

ip netns add $ns
ip netns exec $ns ip link set lo up
ip netns exec $ns sysctl net.ipv6.conf.default.disable_ipv6=0
ip netns exec $ns sysctl net.ipv6.conf.default.forwarding=1

for ((i=0; i<$NIFS; i++))
do
        if=eth$i
        ip netns exec $ns ip link add $if type dummy
        ip netns exec $ns ip link set $if up
        ip netns exec $ns ip -6 addr add 2021:$i::1/120 dev $if
done

for ((i=0; i<$NIFS; i++))
do
        if=eth$i
        ip netns exec $ns ip link del $if
done

ip netns del $ns

Instead of using a fixed msleep(250), this patch tries an extra
rcu_barrier() followed by an exponential backoff.

Time with this patch on a 5.4 kernel:

real	0m7.704s
user	0m0.385s
sys	0m1.230s

Time without this patch:

real    0m31.522s
user    0m0.438s
sys     0m1.156s

v2: use exponential backoff instead of trying to wake up
    netdev_wait_allrefs.
v3: preserve reverse christmas tree ordering of local variables
v4: try an extra rcu_barrier before the backoff, plus some
    cosmetic changes.

Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -10028,10 +10028,12 @@ EXPORT_SYMBOL(netdev_refcnt_read);
  * We can get stuck here if buggy protocols don't correctly
  * call dev_put.
  */
+#define WAIT_REFS_MIN_MSECS 1
+#define WAIT_REFS_MAX_MSECS 250
 static void netdev_wait_allrefs(struct net_device *dev)
 {
 	unsigned long rebroadcast_time, warning_time;
-	int refcnt;
+	int wait = 0, refcnt;
 
 	linkwatch_forget_dev(dev);
 
@@ -10065,7 +10067,13 @@ static void netdev_wait_allrefs(struct net_device *dev)
 			rebroadcast_time = jiffies;
 		}
 
-		msleep(250);
+		if (!wait) {
+			rcu_barrier();
+			wait = WAIT_REFS_MIN_MSECS;
+		} else {
+			msleep(wait);
+			wait = min(wait << 1, WAIT_REFS_MAX_MSECS);
+		}
 
 		refcnt = netdev_refcnt_read(dev);