From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH net-next V3] net: dynamic ingress_queue allocation
Date: Mon, 04 Oct 2010 10:42:09 +0200
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Cc: hadi@cyberus.ca, David Miller <davem@davemloft.net>,
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To: Jarek Poplawski <jarkao2@gmail.com>
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Le dimanche 03 octobre 2010 =C3=A0 11:42 +0200, Jarek Poplawski a =C3=A9=
crit :

> I'd consider rcu_dereference_rtnl(). Btw, technically qdisc_lookup()
> doesn't require rtnl, and there was time it was used without it
> (on xmit path).


Hmm, for me, rcu_dereference_rtnl() is a bit lazy.

Either we are a reader and should use rcu_dereference(), or a writer an=
d
RTNL should be held.

Mixing two conditions in a "super macro" is a workaround that we used t=
o
promptly shutup some lockdep splats. Real fix would be to use strict
lockdep conditions, because this better documents the code and the
locking invariants.

BTW, rtnl_dereference() should be changed to use
rcu_dereference_protected() instead of rcu_dereference_check() :
If RTBL is held, there is no need to force a barrier.


> I think you should also add a comment here why this rcu is used, and
> that it changes only once in dev's liftime.
>=20

This comment was needed in the previous version of the patch, with the
smb_wmb() barrier. Now I switched to regular rcu use, nothing prevents
us to change dev->ingress_queue in flight. Of course there is no curren=
t
interest doing so.