netdev - Re: [PATCH v2] ipv6: acquire write lock for addr_list in dev_forward

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Date:   Mon, 21 Mar 2022 19:15:21 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     David Ahern <dsahern@...nel.org>,
        Niels Dossche <dossche.niels@...il.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH v2] ipv6: acquire write lock for addr_list in
 dev_forward_change

On Mon, 2022-03-21 at 09:42 -0600, David Ahern wrote:
> On 3/19/22 7:17 AM, Niels Dossche wrote:
> > I have an additional question about the locks on the addr_list actually.
> > In addrconf_ifdown, there's a loop on addr_list within a write lock in idev->lock
> > > list_for_each_entry_safe(ifa, tmp, &idev->addr_list, if_list)
> > The loop body unlocks the idev->lock and reacquires it later. I assume because of the lock dependency on ifa->lock and the calls that acquire the mc_lock? Shouldn't that list iteration also be protected during the whole iteration?
> > 
> 
> 
> That loop needs to be improved as well. Locking in ipv6 code is a bit
> hairy.

I *think* we could re-use the if_list_aux trick: create a tmp list
under idev->lock using ifa->if_list_aux and traverse (still using the
_safe variant) such list with no lock.

Still in addrconf_ifdown(), there is a similar loop for
'tempaddr_list'.

In the latter case I think we could splice the idev->lock protected
list into a tmp one and traverse the latter with no lock held.

@Niels: could you look at that, too?

Thanks!

Paolo