netdev - Re: [PATCH net 4/4] tcp: various missing rcu_read_lock around __sk_dst

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Message-ID: <20160401042655.GA16500@ast-mbp.thefacebook.com>
Date:	Thu, 31 Mar 2016 21:26:57 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, sasha.levin@...cle.com,
	daniel@...earbox.net, mkubecek@...e.cz
Subject: Re: [PATCH net 4/4] tcp: various missing rcu_read_lock around
 __sk_dst_get

On Fri, Apr 01, 2016 at 06:12:49AM +0200, Hannes Frederic Sowa wrote:
> On 01.04.2016 06:04, Alexei Starovoitov wrote:
> >On Thu, Mar 31, 2016 at 08:03:38PM -0700, Eric Dumazet wrote:
> >>On Thu, 2016-03-31 at 18:45 -0700, Alexei Starovoitov wrote:
> >>
> >>>Eric, what's your take on Hannes's patch 2 ?
> >>>Is it more accurate to ask lockdep to check for actual lock
> >>>or lockdep can rely on owned flag?
> >>>Potentially there could be races between setting the flag and
> >>>actual lock... but that code is contained, so unlikely.
> >>>Will we find the real issues with this 'stronger' check or
> >>>just spend a ton of time adapting to new model like your other
> >>>patch for release_sock and whatever may need to come next...
> >>
> >>More precise lockdep checks are certainly good, I only objected to 4/4
> >>trying to work around another bug.
> >>
> >>But why do we rush for 'net' tree ?
> >>
> >>This looks net-next material to me.
> >>
> >>Locking changes are often subtle, lets take the time to do them
> >>properly.
> >
> >completely agree. I think only first patch belongs in net.
> >Everything else is net-next material.
> 
> Problem with first patch is that it uses lock_sock_fast, thus the current
> sock_owned_by_user check doesn't get rid the lockdep warning. :/
> 
> Thus we would need to go with the two first patches. Do you think it is
> acceptable? I actually didn't see a problem and testing showed no problems
> so far.

I see. right. the patch 1 only makes sense when coupled with 2.
but now I'm not so sure that lockdep_is_held(&sk->sk_lock.slock)
is a valid check, since current sock_owned_by_user() is equivalent
to lockdep_is_held(&sk->sk_lock) only.
I would go with Daniel's approach. Much simpler to reason about.