[PATCH v3 net-next 09/15] net/sched: sch_pie: annotate data-races in pie_dump_stats()

Eric Dumazet
2026-04-10T18:31:04Z

pie_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_pie_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>
---
include/net/pie.h | 2 +-
net/sched/sch_pie.c | 38 +++++++++++++++++++-------------------
2 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/include/net/pie.h b/include/net/pie.h
index 01cbc66825a40bd21c0a044b1180cbbc346785df..1f3db0c355149b41823a891c9156cac625122031 100644
--- a/include/net/pie.h
+++ b/include/net/pie.h
@@ -104,7 +104,7 @@ static inline void pie_vars_init(struct pie_vars *vars)
vars->dq_tstamp = DTIME_INVALID;
vars->accu_prob = 0;
vars->dq_count = DQCOUNT_INVALID;
- vars->avg_dq_rate = 0;
+ WRITE_ONCE(vars->avg_dq_rate, 0);
}

static inline struct pie_skb_cb *get_pie_cb(const struct sk_buff *skb)
diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c
index 16f3f629cb8e4be71431f7e50a278e3c7fdba8d0..fb53fbf0e328571be72b66ba4e75a938e1963422 100644
--- a/net/sched/sch_pie.c
+++ b/net/sched/sch_pie.c
@@ -90,7 +90,7 @@ static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
bool enqueue = false;

if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
- q->stats.overlimit++;
+ WRITE_ONCE(q->stats.overlimit, q->stats.overlimit + 1);
goto out;
}

@@ -104,7 +104,7 @@ static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
/* If packet is ecn capable, mark it if drop probability
* is lower than 10%, else drop it.
*/
- q->stats.ecn_mark++;
+ WRITE_ONCE(q->stats.ecn_mark, q->stats.ecn_mark + 1);
enqueue = true;
}

@@ -114,15 +114,15 @@ static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
if (!q->params.dq_rate_estimator)
pie_set_enqueue_time(skb);

- q->stats.packets_in++;
+ WRITE_ONCE(q->stats.packets_in, q->stats.packets_in + 1);
if (qdisc_qlen(sch) > q->stats.maxq)
- q->stats.maxq = qdisc_qlen(sch);
+ WRITE_ONCE(q->stats.maxq, qdisc_qlen(sch));

return qdisc_enqueue_tail(skb, sch);
}

out:
- q->stats.dropped++;
+ WRITE_ONCE(q->stats.dropped, q->stats.dropped + 1);
q->vars.accu_prob = 0;
return qdisc_drop_reason(skb, sch, to_free, reason);
}
@@ -267,11 +267,11 @@ void pie_process_dequeue(struct sk_buff *skb, struct pie_params *params,
count = count / dtime;

if (vars->avg_dq_rate == 0)
- vars->avg_dq_rate = count;
+ WRITE_ONCE(vars->avg_dq_rate, count);
else
- vars->avg_dq_rate =
+ WRITE_ONCE(vars->avg_dq_rate,
(vars->avg_dq_rate -
- (vars->avg_dq_rate >> 3)) + (count >> 3);
+ (vars->avg_dq_rate >> 3)) + (count >> 3));

/* If the queue has receded below the threshold, we hold
* on to the last drain rate calculated, else we reset
@@ -381,7 +381,7 @@ void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
if (delta > 0) {
/* prevent overflow */
if (vars->prob < oldprob) {
- vars->prob = MAX_PROB;
+ WRITE_ONCE(vars->prob, MAX_PROB);
/* Prevent normalization error. If probability is at
* maximum value already, we normalize it here, and
* skip the check to do a non-linear drop in the next
@@ -392,7 +392,7 @@ void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
} else {
/* prevent underflow */
if (vars->prob > oldprob)
- vars->prob = 0;
+ WRITE_ONCE(vars->prob, 0);
}

/* Non-linear drop in probability: Reduce drop probability quickly if
@@ -403,7 +403,7 @@ void pie_calculate_probability(struct pie_params *params, struct pie_vars *vars,
/* Reduce drop probability to 98.4% */
vars->prob -= vars->prob / 64;

- vars->qdelay = qdelay;
+ WRITE_ONCE(vars->qdelay, qdelay);
vars->backlog_old = backlog;

/* We restart the measurement cycle if the following conditions are met
@@ -502,21 +502,21 @@ static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
struct pie_sched_data *q = qdisc_priv(sch);
struct tc_pie_xstats st = {
.prob = q->vars.prob << BITS_PER_BYTE,
- .delay = ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
+ .delay = ((u32)PSCHED_TICKS2NS(READ_ONCE(q->vars.qdelay))) /
NSEC_PER_USEC,
- .packets_in = q->stats.packets_in,
- .overlimit = q->stats.overlimit,
- .maxq = q->stats.maxq,
- .dropped = q->stats.dropped,
- .ecn_mark = q->stats.ecn_mark,
+ .packets_in = READ_ONCE(q->stats.packets_in),
+ .overlimit = READ_ONCE(q->stats.overlimit),
+ .maxq = READ_ONCE(q->stats.maxq),
+ .dropped = READ_ONCE(q->stats.dropped),
+ .ecn_mark = READ_ONCE(q->stats.ecn_mark),
};

/* avg_dq_rate is only valid if dq_rate_estimator is enabled */
st.dq_rate_estimating = q->params.dq_rate_estimator;

/* unscale and return dq_rate in bytes per sec */
- if (q->params.dq_rate_estimator)
- st.avg_dq_rate = q->vars.avg_dq_rate *
+ if (st.dq_rate_estimating)
+ st.avg_dq_rate = READ_ONCE(q->vars.avg_dq_rate) *
(PSCHED_TICKS_PER_SEC) >> PIE_SCALE;

return gnet_stats_copy_app(d, &st, sizeof(st));
--
2.53.0.1213.gd9a14994de-goog