[PATCH v2 net] net/sched: sch_fq_codel: annotate data-races from fq_codel_dump_class_stats()

Eric Dumazet
2026-05-04T16:38:45Z

fq_codel_dump_class_stats() acquires qdisc spinlock only when requested
to follow flow->head chain.

As we did in sch_cake recently, add the missing READ_ONCE()/WRITE_ONCE()
annotations.

Fixes: edb09eb17ed8 ("net: sched: do not acquire qdisc spinlock in qdisc/class stats dump")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
v2: added WRITE_ONCE(flow->cvars.count, flow->cvars.count + i);

net/sched/sch_fq_codel.c | 39 ++++++++++++++++++++-------------------
1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index 0664b2f2d6f28041e5250a44fc92311116ae0cf1..24db54684e8a5997b075e0f4072f49779ae45abb 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -117,7 +117,7 @@ static inline struct sk_buff *dequeue_head(struct fq_codel_flow *flow)
{
struct sk_buff *skb = flow->head;

- flow->head = skb->next;
+ WRITE_ONCE(flow->head, skb->next);
skb_mark_not_on_list(skb);
return skb;
}
@@ -127,7 +127,7 @@ static inline void flow_queue_add(struct fq_codel_flow *flow,
struct sk_buff *skb)
{
if (flow->head == NULL)
- flow->head = skb;
+ WRITE_ONCE(flow->head, skb);
else
flow->tail->next = skb;
flow->tail = skb;
@@ -173,8 +173,8 @@ static unsigned int fq_codel_drop(struct Qdisc *sch, unsigned int max_packets,
} while (++i < max_packets && len < threshold);

/* Tell codel to increase its signal strength also */
- flow->cvars.count += i;
- q->backlogs[idx] -= len;
+ WRITE_ONCE(flow->cvars.count, flow->cvars.count + i);
+ WRITE_ONCE(q->backlogs[idx], q->backlogs[idx] - len);
q->memory_usage -= mem;
sch->qstats.drops += i;
sch->qstats.backlog -= len;
@@ -204,13 +204,13 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
codel_set_enqueue_time(skb);
flow = &q->flows[idx];
flow_queue_add(flow, skb);
- q->backlogs[idx] += qdisc_pkt_len(skb);
+ WRITE_ONCE(q->backlogs[idx], q->backlogs[idx] + qdisc_pkt_len(skb));
qdisc_qstats_backlog_inc(sch, skb);

if (list_empty(&flow->flowchain)) {
list_add_tail(&flow->flowchain, &q->new_flows);
q->new_flow_count++;
- flow->deficit = q->quantum;
+ WRITE_ONCE(flow->deficit, q->quantum);
}
get_codel_cb(skb)->mem_usage = skb->truesize;
q->memory_usage += get_codel_cb(skb)->mem_usage;
@@ -263,7 +263,8 @@ static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
flow = container_of(vars, struct fq_codel_flow, cvars);
if (flow->head) {
skb = dequeue_head(flow);
- q->backlogs[flow - q->flows] -= qdisc_pkt_len(skb);
+ WRITE_ONCE(q->backlogs[flow - q->flows],
+ q->backlogs[flow - q->flows] - qdisc_pkt_len(skb));
q->memory_usage -= get_codel_cb(skb)->mem_usage;
sch->q.qlen--;
sch->qstats.backlog -= qdisc_pkt_len(skb);
@@ -296,7 +297,7 @@ static struct sk_buff *fq_codel_dequeue(struct Qdisc *sch)
flow = list_first_entry(head, struct fq_codel_flow, flowchain);

if (flow->deficit <= 0) {
- flow->deficit += q->quantum;
+ WRITE_ONCE(flow->deficit, flow->deficit + q->quantum);
list_move_tail(&flow->flowchain, &q->old_flows);
goto begin;
}
@@ -314,7 +315,7 @@ static struct sk_buff *fq_codel_dequeue(struct Qdisc *sch)
goto begin;
}
qdisc_bstats_update(sch, skb);
- flow->deficit -= qdisc_pkt_len(skb);
+ WRITE_ONCE(flow->deficit, flow->deficit - qdisc_pkt_len(skb));

if (q->cstats.drop_count) {
qdisc_tree_reduce_backlog(sch, q->cstats.drop_count,
@@ -328,7 +329,7 @@ static struct sk_buff *fq_codel_dequeue(struct Qdisc *sch)
static void fq_codel_flow_purge(struct fq_codel_flow *flow)
{
rtnl_kfree_skbs(flow->head, flow->tail);
- flow->head = NULL;
+ WRITE_ONCE(flow->head, NULL);
}

static void fq_codel_reset(struct Qdisc *sch)
@@ -656,21 +657,21 @@ static int fq_codel_dump_class_stats(struct Qdisc *sch, unsigned long cl,

memset(&xstats, 0, sizeof(xstats));
xstats.type = TCA_FQ_CODEL_XSTATS_CLASS;
- xstats.class_stats.deficit = flow->deficit;
+ xstats.class_stats.deficit = READ_ONCE(flow->deficit);
xstats.class_stats.ldelay =
- codel_time_to_us(flow->cvars.ldelay);
- xstats.class_stats.count = flow->cvars.count;
- xstats.class_stats.lastcount = flow->cvars.lastcount;
- xstats.class_stats.dropping = flow->cvars.dropping;
- if (flow->cvars.dropping) {
- codel_tdiff_t delta = flow->cvars.drop_next -
+ codel_time_to_us(READ_ONCE(flow->cvars.ldelay));
+ xstats.class_stats.count = READ_ONCE(flow->cvars.count);
+ xstats.class_stats.lastcount = READ_ONCE(flow->cvars.lastcount);
+ xstats.class_stats.dropping = READ_ONCE(flow->cvars.dropping);
+ if (xstats.class_stats.dropping) {
+ codel_tdiff_t delta = READ_ONCE(flow->cvars.drop_next) -
codel_get_time();

xstats.class_stats.drop_next = (delta >= 0) ?
codel_time_to_us(delta) :
-codel_time_to_us(-delta);
}
- if (flow->head) {
+ if (READ_ONCE(flow->head)) {
sch_tree_lock(sch);
skb = flow->head;
while (skb) {
@@ -679,7 +680,7 @@ static int fq_codel_dump_class_stats(struct Qdisc *sch, unsigned long cl,
}
sch_tree_unlock(sch);
}
- qs.backlog = q->backlogs[idx];
+ qs.backlog = READ_ONCE(q->backlogs[idx]);
qs.drops = 0;
}
if (gnet_stats_copy_queue(d, NULL, &qs, qs.qlen) < 0)
--
2.54.0.545.g6539524ca2-goog