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: e49efd52 ("net: sched: gred: support reporting stats from offloads")
Link: https://lore.kernel.org/r/20230113044137.1383067-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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;