net: sched: Protect Qdisc::bstats with u64_stats

The not-per-CPU variant of qdisc tc (traffic control) statistics,
Qdisc::gnet_stats_basic_packed bstats, is protected with Qdisc::running
sequence counter.

This sequence counter is used for reliably protecting bstats reads from
parallel writes. Meanwhile, the seqcount's write section covers a much
wider area than bstats update: qdisc_run_begin() => qdisc_run_end().

That read/write section asymmetry can lead to needless retries of the
read section. To prepare for removing the Qdisc::running sequence
counter altogether, introduce a u64_stats sync point inside bstats
instead.

Modify _bstats_update() to start/end the bstats u64_stats write
section.

For bisectability, and finer commits granularity, the bstats read
section is still protected with a Qdisc::running read/retry loop and
qdisc_run_begin/end() still starts/ends that seqcount write section.
Once all call sites are modified to use _bstats_update(), the
Qdisc::running seqcount will be removed and bstats read/retry loop will
be modified to utilize the internal u64_stats sync point.

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

[bigeasy: Minor commit message edits, init all gnet_stats_basic_packed.]

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, 3 insertions, 1 deletions

diff --git a/net/sched/sch_gred.c b/net/sched/sch_gred.c
index 621dc6afde8f..2ddcbb2efdbb 100644
--- a/net/sched/sch_gred.c
+++ b/net/sched/sch_gred.c
@@ -364,9 +364,11 @@ static int gred_offload_dump_stats(struct Qdisc *sch)
 	hw_stats->handle = sch->handle;
 	hw_stats->parent = sch->parent;
 
-	for (i = 0; i < MAX_DPs; i++)
+	for (i = 0; i < MAX_DPs; i++) {
+		gnet_stats_basic_packed_init(&hw_stats->stats.bstats[i]);
 		if (table->tab[i])
 			hw_stats->stats.xstats[i] = &table->tab[i]->stats;
+	}
 
 	ret = qdisc_offload_dump_helper(sch, TC_SETUP_QDISC_GRED, hw_stats);
 	/* Even if driver returns failure adjust the stats - in case offload