netdev - Re: WARN due to local_bh_disable called with interrupts disabled

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Date:	Thu, 19 Nov 2015 04:28:32 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	subashab@...eaurora.org, netfilter-devel@...r.kernel.org,
	pablo@...filter.org,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: WARN due to local_bh_disable called with interrupts disabled

On Thu, 2015-11-19 at 11:42 +0100, Jesper Dangaard Brouer wrote:
> On Thu, 19 Nov 2015 02:37:54 -0000 subashab@...eaurora.org wrote:
> 
> > We are seeing a WARN due to local_bh_disable called with interrupts
> > disabled with CONFIG_IRQSOFF_TRACER / CONFIG_PREEMPT_TRACER.
> 
> AFAIK this WARN happens due to a being called from hardware interrupt
> context.  __local_bh_disable_ip calls:  WARN_ON_ONCE(in_irq());
> 
> > Here is the WARN trace
> > 
> >   1833.210427:   <6> Call trace:
> >   1833.212833:   <2> [<ffffffc000088530>] dump_backtrace+0x0/0x270
> >   1833.212838:   <2> [<ffffffc0000887b0>] show_stack+0x10/0x1c
> >   1833.212853:   <2> [<ffffffc000c6edac>] dump_stack+0x74/0xb8
> >   1833.212862:   <2> [<ffffffc0000a0fe4>] warn_slowpath_common+0x88/0xb0
> >   1833.212865:   <2> [<ffffffc0000a10d0>] warn_slowpath_null+0x14/0x20
> >   1833.212870:   <2> [<ffffffc0000a46dc>] __local_bh_disable_ip+0x4c/0xc8
> >   1833.212882:   <2> [<ffffffc000ae2788>] destroy_conntrack+0x90/0x184
> >   1833.212888:   <2> [<ffffffc000adcd50>] nf_conntrack_destroy+0x28/0x38
> >   1833.212896:   <2> [<ffffffc000a97550>] skb_release_head_state+0xa4/0xe0
> >   1833.212900:   <2> [<ffffffc000a977e0>] __kfree_skb+0x10/0xbc
> >   1833.212904:   <2> [<ffffffc000a976bc>] kfree_skb+0xb4/0xdc
> >   1833.212912:   <2> [<ffffffc000aa4660>] flush_backlog+0x88/0x120
> >   1833.212922:   <2> [<ffffffc00010e544>] flush_smp_call_function_queue+0xb4/0x154
> >   1833.212926:   <2> [<ffffffc00010efcc>] generic_smp_call_function_single_interrupt+0xc/0x18
> >   1833.212932:   <2> [<ffffffc000091050>] handle_IPI+0x120/0x338
> >   1833.212937:   <2> [<ffffffc000081580>] gic_handle_irq+0xb8/0xdc
> 
> The call gic_handle_irq() sounds like a hardware IRQ func/context.
> 
> The flush_backlog() call is due to the device is being unregistered. 
> 
> 
> > Here is the callstack which leads to this WARN.
> > 
> > netdev_run_todo
> > on_each_cpu //This disables irq with local_irq_save(flags)
> > flush_backlog
> > kfree_skb
> > ..
> > destroy_conntrack //This disables irq's again through local_bh_disable
> 
> __local_bh_disable_ip() (when CONFIG_TRACE_IRQFLAGS is enabled) calls:
>  raw_local_irq_save(flags);
>  raw_local_irq_restore(flags);
> 
> Thus, it should be safe, as the save/restore variants are used.
> 
> > I noticed that this was introduced by commit ca7433df3a ("netfilter:
> > conntrack: seperate expect locking from nf_conntrack_lock ").
> > 
> > Since interrupts are already disabled when flush_backlog is called, is it
> > expected to disable bottom halves in destroy_conntrack?
> 
> I'm surprised to see kfree_skb() being called from hardirq context, I
> though that was not allowed.
> 
> AFAIK this is the reason we have: __dev_kfree_skb_any() which defer
> freeing the SKB if (in_irq() || irqs_disabled()).
> 
> Code:
>  void __dev_kfree_skb_any(struct sk_buff *skb, enum skb_free_reason reason)
>  {
> 	if (in_irq() || irqs_disabled())
> 		__dev_kfree_skb_irq(skb, reason);
> 	else
> 		dev_kfree_skb(skb);
>  }

Right, but flush_backlog() is processing packets coming from RX, that
should have no conntracking attached at all.

Might be a bug in a tunnel ?



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