netdev - Re: [PATCH net] ipv6: fix RTNL assert fail in DAD

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Message-ID: <20140318175406.78339ffe@nehalam.linuxnetplumber.net>
Date:	Tue, 18 Mar 2014 17:54:06 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net] ipv6: fix RTNL assert fail in DAD

On Tue, 18 Mar 2014 01:29:08 +0100
Hannes Frederic Sowa <hannes@...essinduktion.org> wrote:

> Hi!
> 
> On Mon, Mar 17, 2014 at 04:18:53PM -0700, Stephen Hemminger wrote:
> > IPv6 duplicate address detection is triggering the following assertion
> > failure when using macvlan + vif + multicast.
> >  RTNL: assertion failed at net/core/dev.c (4496)
> > 
> > This happens because the DAD timer is adding a multicast address without
> > acquiring the RTNL mutex. In order to acquire the RTNL mutex, it must be
> > done in process context; therefore it must be in a workqueue.
> > 
> > Full backtrace:
> > [  541.030090] RTNL: assertion failed at net/core/dev.c (4496)
> > [  541.031143] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G           O 3.10.33-1-amd64-vyatta #1
> > [  541.031145] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
> > [  541.031146]  ffffffff8148a9f0 000000000000002f ffffffff813c98c1 ffff88007c4451f8
> > [  541.031148]  0000000000000000 0000000000000000 ffffffff813d3540 ffff88007fc03d18
> > [  541.031150]  0000880000000006 ffff88007c445000 ffffffffa0194160 0000000000000000
> > [  541.031152] Call Trace:
> > [  541.031153]  <IRQ>  [<ffffffff8148a9f0>] ? dump_stack+0xd/0x17
> > [  541.031180]  [<ffffffff813c98c1>] ? __dev_set_promiscuity+0x101/0x180
> > [  541.031183]  [<ffffffff813d3540>] ? __hw_addr_create_ex+0x60/0xc0
> > [  541.031185]  [<ffffffff813cfe1a>] ? __dev_set_rx_mode+0xaa/0xc0
> > [  541.031189]  [<ffffffff813d3a81>] ? __dev_mc_add+0x61/0x90
> > [  541.031198]  [<ffffffffa01dcf9c>] ? igmp6_group_added+0xfc/0x1a0 [ipv6]
> > [  541.031208]  [<ffffffff8111237b>] ? kmem_cache_alloc+0xcb/0xd0
> > [  541.031212]  [<ffffffffa01ddcd7>] ? ipv6_dev_mc_inc+0x267/0x300 [ipv6]
> > [  541.031216]  [<ffffffffa01c2fae>] ? addrconf_join_solict+0x2e/0x40 [ipv6]
> > [  541.031219]  [<ffffffffa01ba2e9>] ? ipv6_dev_ac_inc+0x159/0x1f0 [ipv6]
> > [  541.031223]  [<ffffffffa01c0772>] ? addrconf_join_anycast+0x92/0xa0 [ipv6]
> > [  541.031226]  [<ffffffffa01c311e>] ? __ipv6_ifa_notify+0x11e/0x1e0 [ipv6]
> > [  541.031229]  [<ffffffffa01c3213>] ? ipv6_ifa_notify+0x33/0x50 [ipv6]
> 
> This is the most often case but I fear there are more of them.
> 
> addrconf_verify seems unsafe, too, when removing the last ipv6 address. So
> does addrconf_prefix_rcv if adding first address.
> 
> I wonder if we should put the whole ipv6_ifa_notify infrastructure in a
> workqueue? I don't like that either and it could add subtile races.

That is option, might be some call chains that already have rtnl_lock held.

> 
> Those races also seem possible if we only defer addrconf_join_solict,
> addrconf_leave_solict, addrconf_join_anycast and addrconf_leave_anycast to
> workqueues.
> 
> This change is certainly going into the right direction but I am not sure if
> we could generalize it.

There is a lot of bookkeeping that happens for cases where nothing
changes at the device layer. Want to avoid rtnl_lock unless there is
something that is going to happen.

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