From 67c9e6270f3013e4d86ec57c4e7f27459f2a0652 Mon Sep 17 00:00:00 2001 From: "Ahmed S. Darwish" Date: Sat, 16 Oct 2021 10:49:07 +0200 Subject: 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 Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: David S. Miller --- net/sched/sch_gred.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'net/sched/sch_gred.c') 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 -- cgit v1.2.3