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Date:	Thu, 24 May 2012 10:16:23 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Arvid Brodin <Arvid.Brodin@...n.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Bruno Ferreira <balferreira@...glemail.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC] net/hsr: Add support for IEC 62439-3 High-availability
 Seamless Redundancy

On Thu, 24 May 2012 17:09:53 +0000
Arvid Brodin <Arvid.Brodin@...n.com> wrote:

> On 2012-05-14 20:28, Stephen Hemminger wrote:
> > On Mon, 14 May 2012 18:11:44 +0000
> > Arvid Brodin <Arvid.Brodin@...n.com> wrote:
> > 
> >> On 2012-03-27 15:20, Arvid Brodin wrote:
> >>> Hi!
> >>
> >> *snip*
> >>>
> >>> 2) I have a locking problem that I haven't managed to figure out. This happens
> >>>    the first time I send any packet (hsr_dev_xmit() in hsr_device.c:121, called
> >>>    from hsr_device.c:147). It happens even if I set skb2 to NULL (i.e. only send
> >>>    one copy):
> >>>
> >>> =============================================
> >>> [ INFO: possible recursive locking detected ]
> >>> 2.6.37 #118
> >>> ---------------------------------------------
> >>> swapper/0 is trying to acquire lock:
> >>>  (_xmit_ETHER#2){+.-...}, at: [<901bf38e>] sch_direct_xmit+0x24/0x152
> >>>
> >>> but task is already holding lock:
> >>>  (_xmit_ETHER#2){+.-...}, at: [<901b4d1a>] dev_queue_xmit+0x31e/0x3cc
> >>>
> >>> other info that might help us debug this:
> >>> 4 locks held by swapper/0:
> >>>  #0:  (&n->timer){+.-...}, at: [<9002bc20>] run_timer_softirq+0x98/0x184
> >>>  #1:  (rcu_read_lock_bh){.+....}, at: [<901b49fc>] dev_queue_xmit+0x0/0x3cc
> >>>  #2:  (_xmit_ETHER#2){+.-...}, at: [<901b4d1a>] dev_queue_xmit+0x31e/0x3cc
> >>>  #3:  (rcu_read_lock_bh){.+....}, at: [<901b49fc>] dev_queue_xmit+0x0/0x3cc
> >>>
> >>> stack backtrace:
> >>> Call trace:
> >>>  [<9001c640>] dump_stack+0x18/0x20
> >>>  [<90040eac>] validate_chain+0x40c/0x9ac
> >>>  [<90041a58>] __lock_acquire+0x60c/0x670
> >>>  [<90042f32>] lock_acquire+0x3a/0x48
> >>>  [<902201a4>] _raw_spin_lock+0x20/0x44
> >>>  [<901bf38e>] sch_direct_xmit+0x24/0x152
> >>>  [<901b4c14>] dev_queue_xmit+0x218/0x3cc
> >>>  [<9021c2e0>] slave_xmit+0x10/0x14
> >>>  [<9021c540>] hsr_dev_xmit+0x88/0x8c
> >>>  [<901b4942>] dev_hard_start_xmit+0x3c6/0x480
> >>>  [<901b4d34>] dev_queue_xmit+0x338/0x3cc
> >>>  [<901e3cd8>] arp_xmit+0x8/0xc
> >>>  [<901e4436>] arp_send+0x2a/0x2c
> >>>  [<901e4e74>] arp_solicit+0x15c/0x170
> >>>  [<901bad0c>] neigh_timer_handler+0x1c0/0x204
> >>>  [<9002bc8a>] run_timer_softirq+0x102/0x184
> >>>  [<900287d8>] __do_softirq+0x64/0xe0
> >>>  [<9002896a>] do_softirq+0x26/0x48
> >>>  [<90028a66>] irq_exit+0x2e/0x64
> >>>  [<90019f16>] do_IRQ+0x46/0x5c
> >>>  [<90018428>] irq_level0+0x18/0x60
> >>>  [<9021cc16>] rest_init+0x72/0x98
> >>>  [<9000063c>] start_kernel+0x21c/0x258
> >>>  [<00000000>] 0x0
> >>>
> >>>    Any idea why this happens? I need help!
> >>
> >>
> >> I've spent a few days digging into this and the key apparently is NETIF_F_LLTX.
> >>
> >> The problem seems to be that HARD_TX_LOCK is called more than once, first for my virtual
> >> hsr device and then, recursively, for each of the slaves in turn. (At least that's where
> >> lockdep complains - at __netif_tx_lock(), that is.)
> >>
> >> At first I just could not understand why both the VLAN and the bonding code got away with
> >> recursive calls to dev_queue_xmit() but I didn't. After some gooling (a lot, actually) I
> >> found some references to the NETIF_F_LLTX flag (here's one:
> >> http://lwn.net/Articles/121566/). I realised both VLAN and bonding code set this flag. And
> >> sure enough, if I set it for my hsr device lockdep does not complain any more.
> >>
> >> But NETIF_F_LLTX is described as deprecated in both netdevice.h and in
> >> Documentation/networking/netdevices.txt. Is there an alternative solution that I should
> >> use instead?
> >>
> >> (To recap, a hsr device is a virtual device which uses two Ethernet devices as slaves.
> >> This gives redundancy with instant failover, and since nodes are connected in a ring
> >> topology, uses less cabling than duplication.)
> >>
> > 
> > LLTX is deprecated (ie should not be used) for physical devices.
> > 
> > Also, for virtual devices, there should be non transmit queue, this
> > causes mulit-queue lockless semantics to be preserved as the call passes
> > through the virtual device.
> 
> (First: apologies for my late reply; your emails doesn't get through to me for some reason.)
> 
> So does this mean that it is OK to use LLTX for virtual devices? My virtual device has
> zero queue length (no transmit queue), but since it calls dev_queue_xmit for its slaves, I
> still get recursive locking if I don't set LLTX (just like vlan and bonding does).

Yes LLTX is fine for virtual devices.

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