Re: rcu: Add might_sleep() check to synchronize_rcu()

[Thomas Gleixner]

On Fri, 23 Mar 2018, Steven Rostedt wrote:
> On Fri, 23 Mar 2018 22:33:29 +0100 (CET)
> [  150.741223]  [<ffffffff81290287>] synchronize_rcu+0x27/0x90
> [  150.746908]  [<ffffffff83588b35>] __l2tp_session_unhash+0x3d5/0x550
> 
> Looks like __l2tp_session_unhash() is the real culprit here.

Yes. I reported that to netdev already.

> [  150.753281]  [<ffffffff8358891f>] ? __l2tp_session_unhash+0x1bf/0x550
> [  150.759828]  [<ffffffff8114596a>] ? __local_bh_enable_ip+0x6a/0xd0
> [  150.766123]  [<ffffffff8358ddb0>] ? l2tp_udp_encap_recv+0xd90/0xd90
> [  150.772497]  [<ffffffff83588e97>] l2tp_tunnel_closeall+0x1e7/0x3a0
> [  150.778782]  [<ffffffff835897be>] l2tp_tunnel_destruct+0x30e/0x5a0
> [  150.785067]  [<ffffffff8358965a>] ? l2tp_tunnel_destruct+0x1aa/0x5a0
> [  150.791537]  [<ffffffff835894b0>] ? l2tp_tunnel_del_work+0x460/0x460
> [  150.797997]  [<ffffffff82ee8053>] __sk_destruct+0x53/0x570
> [  150.803588]  [<ffffffff81293918>] rcu_process_callbacks+0x898/0x1300
> [  150.810048]  [<ffffffff812939f7>] ? rcu_process_callbacks+0x977/0x1300
> [  150.816684]  [<ffffffff82ee8000>] ? __sk_dst_check+0x240/0x240
> [  150.822625]  [<ffffffff838be5d6>] __do_softirq+0x206/0x951
> [  150.828223]  [<ffffffff81147315>] irq_exit+0x165/0x190
> [  150.833557]  [<ffffffff838bd1eb>] smp_apic_timer_interrupt+0x7b/0xa0
> [  150.840018]  [<ffffffff838b9470>] apic_timer_interrupt+0xa0/0xb0
> [  150.846132]  <EOI> [  150.848166]  [<ffffffff838b6756>] ? native_safe_halt+0x6/0x10
> [  150.854036]  [<ffffffff8123bf2d>] ? trace_hardirqs_on+0xd/0x10
> [  150.859973]  [<ffffffff838b5d85>] default_idle+0x55/0x360
> [  150.865478]  [<ffffffff8106be0a>] arch_cpu_idle+0xa/0x10
> 
> I think you want this instead, as __l2tp_session_unhash is what looks
> like might be hiding the call to synchronize_rcu(). It's not called in
> all instances, and I don't think your patch would have triggered the
> issues before hand. You want this:
>
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 194a7483bb93..857b494bee29 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1677,6 +1677,8 @@ void __l2tp_session_unhash(struct l2tp_session *session)
>  {
>  	struct l2tp_tunnel *tunnel = session->tunnel;
>  
> +	might_sleep();
> +
>  	/* Remove the session from core hashes */
>  	if (tunnel) {
>  		/* Remove from the per-tunnel hash */

That too :)