net: frag, avoid several CPUs grabbing same frag queue during LRU evictor loop

The LRU list is protected by its own lock, since commit 3ef0eb0d
(net: frag, move LRU list maintenance outside of rwlock), and
no-longer by a read_lock.

This makes it possible, to remove the inet_frag_queue, which is about
to be "evicted", from the LRU list head.  This avoids the problem, of
several CPUs grabbing the same frag queue.

Note, cannot remove the inet_frag_lru_del() call in fq_unlink()
called by inet_frag_kill(), because inet_frag_kill() is also used in
other situations.  Thus, we use list_del_init() to allow this
double list_del to work.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inet_fragment.c

@@ -204,6 +204,9 @@ int inet_frag_evictor(struct netns_frags *nf, struct inet_frags *f, bool force)
 		q = list_first_entry(&nf->lru_list,
 				struct inet_frag_queue, lru_list);
 		atomic_inc(&q->refcnt);
+		/* Remove q from list to avoid several CPUs grabbing it */
+		list_del_init(&q->lru_list);
+
 		spin_unlock(&nf->lru_lock);
 
 		spin_lock(&q->lock);