net: sched: Remove Qdisc::running sequence counter

The Qdisc::running sequence counter has two uses:

  1. Reliably reading qdisc's tc statistics while the qdisc is running
     (a seqcount read/retry loop at gnet_stats_add_basic()).

  2. As a flag, indicating whether the qdisc in question is running
     (without any retry loops).

For the first usage, the Qdisc::running sequence counter write section,
qdisc_run_begin() => qdisc_run_end(), covers a much wider area than what
is actually needed: the raw qdisc's bstats update. A u64_stats sync
point was thus introduced (in previous commits) inside the bstats
structure itself. A local u64_stats write section is then started and
stopped for the bstats updates.

Use that u64_stats sync point mechanism for the bstats read/retry loop
at gnet_stats_add_basic().

For the second qdisc->running usage, a __QDISC_STATE_RUNNING bit flag,
accessed with atomic bitops, is sufficient. Using a bit flag instead of
a sequence counter at qdisc_run_begin/end() and qdisc_is_running() leads
to the SMP barriers implicitly added through raw_read_seqcount() and
write_seqcount_begin/end() getting removed. All call sites have been
surveyed though, and no required ordering was identified.

Now that the qdisc->running sequence counter is no longer used, remove
it.

Note, using u64_stats implies no sequence counter protection for 64-bit
architectures. This can lead to the qdisc tc statistics "packets" vs.
"bytes" values getting out of sync on rare occasions. The individual
values will still be valid.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/sched/sch_prio.c b/net/sched/sch_prio.c
index 03fdf31ccb6a..3b8d7197c06b 100644
--- a/net/sched/sch_prio.c
+++ b/net/sched/sch_prio.c
@@ -361,8 +361,8 @@ static int prio_dump_class_stats(struct Qdisc *sch, unsigned long cl,
 	struct Qdisc *cl_q;
 
 	cl_q = q->queues[cl - 1];
-	if (gnet_stats_copy_basic(qdisc_root_sleeping_running(sch),
-				  d, cl_q->cpu_bstats, &cl_q->bstats) < 0 ||
+	if (gnet_stats_copy_basic(d, cl_q->cpu_bstats,
+				  &cl_q->bstats, true) < 0 ||
 	    qdisc_qstats_copy(d, cl_q) < 0)
 		return -1;