net: sched: gred: prevent races when adding offloads to stats

Naresh reports seeing a warning that gred is calling
u64_stats_update_begin() with preemption enabled.
Arnd points out it's coming from _bstats_update().

We should be holding the qdisc lock when writing
to stats, they are also updated from the datapath.

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Link: https://lore.kernel.org/all/CA+G9fYsTr9_r893+62u6UGD3dVaCE-kN9C-Apmb2m=hxjc1Cqg@mail.gmail.com/
Fixes: e49efd5288bd ("net: sched: gred: support reporting stats from offloads")
Link: https://lore.kernel.org/r/20230113044137.1383067-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_gred.c b/net/sched/sch_gred.c
index a661b06..872d127 100644 (file)
--- a/net/sched/sch_gred.c
+++ b/net/sched/sch_gred.c
@@ -377,6 +377,7 @@ static int gred_offload_dump_stats(struct Qdisc *sch)
        /* Even if driver returns failure adjust the stats - in case offload
         * ended but driver still wants to adjust the values.
         */
+       sch_tree_lock(sch);
        for (i = 0; i < MAX_DPs; i++) {
                if (!table->tab[i])
                        continue;
@@ -393,6 +394,7 @@ static int gred_offload_dump_stats(struct Qdisc *sch)
                sch->qstats.overlimits += hw_stats->stats.qstats[i].overlimits;
        }
        _bstats_update(&sch->bstats, bytes, packets);
+       sch_tree_unlock(sch);
 
        kfree(hw_stats);
        return ret;