net_sched: sch_sfq: annotate data-races around q->perturb_period

sfq_perturbation() reads q->perturb_period locklessly.
Add annotations to fix potential issues.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240430180015.3111398-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_sfq.c

@@ -608,6 +608,7 @@ static void sfq_perturbation(struct timer_list *t)
 	struct Qdisc *sch = q->sch;
 	spinlock_t *root_lock;
 	siphash_key_t nkey;
+	int period;
 
 	get_random_bytes(&nkey, sizeof(nkey));
 	rcu_read_lock();
@@ -618,8 +619,12 @@ static void sfq_perturbation(struct timer_list *t)
 		sfq_rehash(sch);
 	spin_unlock(root_lock);
 
-	if (q->perturb_period)
-		mod_timer(&q->perturb_timer, jiffies + q->perturb_period);
+	/* q->perturb_period can change under us from
+	 * sfq_change() and sfq_destroy().
+	 */
+	period = READ_ONCE(q->perturb_period);
+	if (period)
+		mod_timer(&q->perturb_timer, jiffies + period);
 	rcu_read_unlock();
 }
 
@@ -662,7 +667,7 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt)
 		q->quantum = ctl->quantum;
 		q->scaled_quantum = SFQ_ALLOT_SIZE(q->quantum);
 	}
-	q->perturb_period = ctl->perturb_period * HZ;
+	WRITE_ONCE(q->perturb_period, ctl->perturb_period * HZ);
 	if (ctl->flows)
 		q->maxflows = min_t(u32, ctl->flows, SFQ_MAX_FLOWS);
 	if (ctl->divisor) {
@@ -724,7 +729,7 @@ static void sfq_destroy(struct Qdisc *sch)
 	struct sfq_sched_data *q = qdisc_priv(sch);
 
 	tcf_block_put(q->block);
-	q->perturb_period = 0;
+	WRITE_ONCE(q->perturb_period, 0);
 	del_timer_sync(&q->perturb_timer);
 	sfq_free(q->ht);
 	sfq_free(q->slots);