[PATCH net-next 06/15] net/sched: sch_sfb: annotate data-races in sfb_dump_stats()

Eric Dumazet
2026-04-08T12:56:25Z

sfb_dump_stats() only runs with RTNL held,
reading fields that can be changed in qdisc fast path.

Add READ_ONCE()/WRITE_ONCE() annotations.

Alternative would be to acquire the qdisc spinlock, but our long-term
goal is to make qdisc dump operations lockless as much as we can.

tc_sfb_xstats fields don't need to be latched atomically,
otherwise this bug would have been caught earlier.

Fixes: edb09eb17ed8 ("net: sched: do not acquire qdisc spinlock in qdisc/class stats dump")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
net/sched/sch_sfb.c | 46 +++++++++++++++++++++++++++------------------
1 file changed, 28 insertions(+), 18 deletions(-)

diff --git a/net/sched/sch_sfb.c b/net/sched/sch_sfb.c
index 2258567cbcaf70863eace85d347efda882a00145..315edd7f87fcf1600d69a3a92733ddb9fee55e99 100644
--- a/net/sched/sch_sfb.c
+++ b/net/sched/sch_sfb.c
@@ -202,11 +202,14 @@ static u32 sfb_compute_qlen(u32 *prob_r, u32 *avgpm_r, const struct sfb_sched_da
const struct sfb_bucket *b = &q->bins[q->slot].bins[0][0];

for (i = 0; i < SFB_LEVELS * SFB_NUMBUCKETS; i++) {
- if (qlen < b->qlen)
- qlen = b->qlen;
- totalpm += b->p_mark;
- if (prob < b->p_mark)
- prob = b->p_mark;
+ u32 b_qlen = READ_ONCE(b->qlen);
+ u32 b_mark = READ_ONCE(b->p_mark);
+
+ if (qlen < b_qlen)
+ qlen = b_qlen;
+ totalpm += b_mark;
+ if (prob < b_mark)
+ prob = b_mark;
b++;
}
*prob_r = prob;
@@ -295,7 +298,8 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,

if (unlikely(sch->q.qlen >= q->limit)) {
qdisc_qstats_overlimit(sch);
- q->stats.queuedrop++;
+ WRITE_ONCE(q->stats.queuedrop,
+ q->stats.queuedrop + 1);
goto drop;
}

@@ -348,7 +352,8 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,

if (unlikely(minqlen >= q->max)) {
qdisc_qstats_overlimit(sch);
- q->stats.bucketdrop++;
+ WRITE_ONCE(q->stats.bucketdrop,
+ q->stats.bucketdrop + 1);
goto drop;
}

@@ -374,7 +379,8 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
}
if (sfb_rate_limit(skb, q)) {
qdisc_qstats_overlimit(sch);
- q->stats.penaltydrop++;
+ WRITE_ONCE(q->stats.penaltydrop,
+ q->stats.penaltydrop + 1);
goto drop;
}
goto enqueue;
@@ -390,14 +396,17 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
* In either case, we want to start dropping packets.
*/
if (r < (p_min - SFB_MAX_PROB / 2) * 2) {
- q->stats.earlydrop++;
+ WRITE_ONCE(q->stats.earlydrop,
+ q->stats.earlydrop + 1);
goto drop;
}
}
if (INET_ECN_set_ce(skb)) {
- q->stats.marked++;
+ WRITE_ONCE(q->stats.marked,
+ q->stats.marked + 1);
} else {
- q->stats.earlydrop++;
+ WRITE_ONCE(q->stats.earlydrop,
+ q->stats.earlydrop + 1);
goto drop;
}
}
@@ -410,7 +419,8 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
qdisc_qlen_inc(sch);
increment_qlen(&cb, q);
} else if (net_xmit_drop_count(ret)) {
- q->stats.childdrop++;
+ WRITE_ONCE(q->stats.childdrop,
+ q->stats.childdrop + 1);
qdisc_qstats_drop(sch);
}
return ret;
@@ -599,12 +609,12 @@ static int sfb_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
{
struct sfb_sched_data *q = qdisc_priv(sch);
struct tc_sfb_xstats st = {
- .earlydrop = q->stats.earlydrop,
- .penaltydrop = q->stats.penaltydrop,
- .bucketdrop = q->stats.bucketdrop,
- .queuedrop = q->stats.queuedrop,
- .childdrop = q->stats.childdrop,
- .marked = q->stats.marked,
+ .earlydrop = READ_ONCE(q->stats.earlydrop),
+ .penaltydrop = READ_ONCE(q->stats.penaltydrop),
+ .bucketdrop = READ_ONCE(q->stats.bucketdrop),
+ .queuedrop = READ_ONCE(q->stats.queuedrop),
+ .childdrop = READ_ONCE(q->stats.childdrop),
+ .marked = READ_ONCE(q->stats.marked),
};

st.maxqlen = sfb_compute_qlen(&st.maxprob, &st.avgprob, q);
--
2.53.0.1213.gd9a14994de-goog