On Thu, Jan 07, 2016 at 07:52:53AM -0800, Eric Dumazet wrote:
> On Thu, 2016-01-07 at 10:32 -0500, Dave Jones wrote:
> > ===
> > [ INFO: suspicious RCU usage. ]
> > 4.4.0-rc8-firewall+ #1 Not tainted
> > ---
> > net/ipv6/tcp_ipv6.c:465 suspicious rcu_dereference_check() usage!
> >
> > other info that might help us debug this:
> >
> >
> > rcu_scheduler_active = 1, debug_locks = 1
> > 1 lock held by swapper/1/0:
> > #0: (((>rsk_timer))){+.-...}, at: []
> > call_timer_fn+0x5/0x3f0
> >
> > stack backtrace:
> > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.4.0-rc8-firewall+ #1
> > 8801d7a07b28 9948b3b5 8801d6046500
> > 8801d7a07b58 990e9b7a 8801cd356240
> > 8801d23b1698 8801d23b0c40 8801d7a07ba8 99b635d2
> > Call Trace:
> >[] dump_stack+0x4e/0x79
> > [] lockdep_rcu_suspicious+0xea/0x110
> > [] tcp_v6_send_synack+0x2c2/0x350
> > [] tcp_rtx_synack+0xdd/0x180
> > [] ? tcp_send_probe0+0x1a0/0x1a0
> > [] reqsk_timer_handler+0x4c4/0x530
> > [] ? inet_csk_reqsk_queue_drop+0x3a0/0x3a0
> > [] ? __lock_is_held+0x9b/0xd0
> > [] call_timer_fn+0x132/0x3f0
> > [] ? call_timer_fn+0x5/0x3f0
> > [] ? inet_csk_reqsk_queue_drop+0x3a0/0x3a0
> > [] ? process_timeout+0x10/0x10
> > [] ? trace_hardirqs_on_caller+0x192/0x2a0
> > [] ? preempt_count_sub+0x1a/0x130
> > [] run_timer_softirq+0x47b/0x590
> > [] ? inet_csk_reqsk_queue_drop+0x3a0/0x3a0
> > [] ? internal_add_timer+0x110/0x110
> > [] ? __lock_is_held+0x28/0xd0
> > [] __do_softirq+0x1b2/0x5c0
> > [] irq_exit+0xfc/0x110
> > [] smp_apic_timer_interrupt+0x5f/0x70
> > [] apic_timer_interrupt+0x8b/0x90
> >[] ? cpuidle_enter_state+0x1c7/0x460
> > [] ? cpuidle_enter_state+0x1c2/0x460
> > [] ? rcu_eqs_enter_common+0x139/0x280
> > [] cpuidle_enter+0x17/0x20
> > [] cpu_startup_entry+0x4d2/0x5b0
> > [] ? default_idle_call+0x60/0x60
> > [] ? clockevents_config_and_register+0x64/0x70
> > [] ? setup_APIC_timer+0x115/0x120
> > [] start_secondary+0x23a/0x2a0
> > [] ? set_cpu_sibling_map+0x9c0/0x9c0
> >
>
> Apparently RCU lockdep thinks a softirq handler (timer soft irq) needs
> rcu_read_lock() before rcu_dereference()
>
> This is not clear if this is a lockdep false positive.
>
> Paul, can you remind me why it is needed, as a softirq handler is not
> allowed to schedule or be preempted ?
Hello, Eric!
If this were rcu_dereference_bh(), then you would be OK as is, given that
you are in a softirq handler. But for rcu_dereference(), lockdep does
indeed insist on an rcu_read_lock(). Yes, you would in fact be OK with
the current implementation (I think, anyway), even with preemptible RCU,
but that is an accident of implementation.
Is the required rcu_read_lock() and rcu_read_unlock() resulting in a
performance problem?
Thanx, Paul
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 6b8a8a9091fa..bd100b47c717 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -462,8 +462,10 @@ static int tcp_v6_send_synack(const struct sock *sk,
> struct dst_entry *dst,
> if (np->repflow && ireq->pktopts)
> fl6->flowlabel = ip6_flowlabel(ipv6_hdr(ireq->pktopts));
>
> + rcu_read_lock();
> err = ip6_xmit(sk, skb, fl6, rcu_dereference(np->opt),
> np->tclass);
> + rcu_read_unlock();
> err = net_xmit_eval(err);
> }
>
>
>
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