inet: frags: remove INET_FRAG_EVICTED and use list_evictor for the test

We can simply remove the INET_FRAG_EVICTED flag to avoid all the flags
race conditions with the evictor and use a participation test for the
evictor list, when we're at that point (after inet_frag_kill) in the
timer there're 2 possible cases:

1. The evictor added the entry to its evictor list while the timer was
waiting for the chainlock
or
2. The timer unchained the entry and the evictor won't see it

In both cases we should be able to see list_evictor correctly due
to the sync on the chainlock.

Joint work with Florian Westphal.

Tested-by: Frank Schreuder <[e-mail]>
Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_frag.h

···
  * @INET_FRAG_FIRST_IN: first fragment has arrived
  * @INET_FRAG_LAST_IN: final fragment has arrived
  * @INET_FRAG_COMPLETE: frag queue has been processed and is due for destruction
- * @INET_FRAG_EVICTED: frag queue is being evicted
  */
 enum {
 	INET_FRAG_FIRST_IN	= BIT(0),
 	INET_FRAG_LAST_IN	= BIT(1),
 	INET_FRAG_COMPLETE	= BIT(2),
-	INET_FRAG_EVICTED	= BIT(3)
 };
 
 /**
···
 {
 	if (atomic_dec_and_test(&q->refcnt))
 		inet_frag_destroy(q, f);
+}
+
+static inline bool inet_frag_evicting(struct inet_frag_queue *q)
+{
+	return !hlist_unhashed(&q->list_evictor);
 }
 
 /* Memory Tracking Functions. */

net/ipv4/inet_fragment.c

···
 		if (!del_timer(&fq->timer))
 			continue;
 
-		fq->flags |= INET_FRAG_EVICTED;
 		hlist_add_head(&fq->list_evictor, &expired);
 		++evicted;
 	}

net/ipv4/ip_fragment.c

···
 	ipq_kill(qp);
 	IP_INC_STATS_BH(net, IPSTATS_MIB_REASMFAILS);
 
-	if (!(qp->q.flags & INET_FRAG_EVICTED)) {
+	if (!inet_frag_evicting(&qp->q)) {
 		struct sk_buff *head = qp->q.fragments;
 		const struct iphdr *iph;
 		int err;

net/ipv6/reassembly.c

···
 
 	IP6_INC_STATS_BH(net, __in6_dev_get(dev), IPSTATS_MIB_REASMFAILS);
 
-	if (fq->q.flags & INET_FRAG_EVICTED)
+	if (inet_frag_evicting(&fq->q))
 		goto out_rcu_unlock;
 
 	IP6_INC_STATS_BH(net, __in6_dev_get(dev), IPSTATS_MIB_REASMTIMEOUT);