Re: [Patch net] net_sched: fix a missing rcu barrier in mini_qdisc_pair_swap()

[Jiri Pirko <jiri@resnulli.us>]

Thu, Dec 21, 2017 at 08:26:24AM CET, xiyou.wangcong@gmail.com wrote:
>The rcu_barrier_bh() in mini_qdisc_pair_swap() is to wait for
>flying RCU callback installed by a previous mini_qdisc_pair_swap(),
>however we miss it on the tp_head==NULL path, which leads to that
>the RCU callback still uses miniq_old->rcu after it is freed together
>with qdisc in qdisc_graft(). So just add it on that path too.
>
>Fixes: 46209401f8f6 ("net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath ")

This fixes:
752fbcc33405 ("net_sched: no need to free qdisc in RCU callback")

Before that, the issue was not there as the qdisc struct got removed
after a grace period.


>Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
>Tested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
>Cc: Jiri Pirko <jiri@mellanox.com>
>Cc: John Fastabend <john.fastabend@gmail.com>
>Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
>---
> net/sched/sch_generic.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>index cd1b200acae7..661c7144b53a 100644
>--- a/net/sched/sch_generic.c
>+++ b/net/sched/sch_generic.c
>@@ -1040,6 +1040,8 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
> 
> 	if (!tp_head) {
> 		RCU_INIT_POINTER(*miniqp->p_miniq, NULL);
>+		/* Wait for flying RCU callback before it is freed. */
>+		rcu_barrier_bh();


> 		return;
> 	}
> 
>@@ -1055,7 +1057,7 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
> 	rcu_assign_pointer(*miniqp->p_miniq, miniq);
> 
> 	if (miniq_old)
>-		/* This is counterpart of the rcu barrier above. We need to
>+		/* This is counterpart of the rcu barriers above. We need to

This is incorrect. Here we block in order to not use the same miniq
again in scenario

miniq1 (X)
miniq2
miniq1 (yet there are reader using X)

This call_rcu has 0 relation to the barrier you are adding.


But again, we don't we just free qdisc in call_rcu and avoid the
barrier?