On Mon, Aug 26, 2019 at 03:12:20PM +0000, Bernard Metzler wrote: > > >To: "Bernard Metzler" <bmt@xxxxxxxxxxxxxx> > >From: "Jason Gunthorpe" <jgg@xxxxxxxx> > >Date: 08/26/2019 04:25PM > >Cc: linux-rdma@xxxxxxxxxxxxxxx, bvanassche@xxxxxxx, > >dledford@xxxxxxxxxx > >Subject: [EXTERNAL] Re: [PATCH] RDMA/siw: Fix IPv6 addr_list locking > > > >On Mon, Aug 26, 2019 at 04:17:40PM +0200, Bernard Metzler wrote: > >> Walking the address list of an inet6_dev requires > >> appropriate locking. Since the called function > >> siw_listen_address() may sleep, we have to use > >> rtnl_lock() + rcu_read_lock_bh() instead of > >> read_lock_bh(). > > > >What is the RCU for if you have RTNL? > > > > Frankly, I looked around in net/ipv6 and found, if not > rwlocked, addr_list walking to be rcu protected, even > if rtnl_lock()'d (e.g. addrconf_verify_rtnl()). > > You are saying this is useless and overdone, since all > changes to that list are rtnl_lock protected right? > I was not sure about that. I'm not sure, I thought it was the case that rtnl protected the address lists on write. > For the IPv4 case further up, we also take the rtnl_lock, > and RCU-deref the address pointer (via > in_dev_for_each_ifa_rtnl()). That uses rtnl_derference which calls into rcu_dereference_protected which is saying 'this RCU protected data is being read outside RCU because we are holding the write side lock' Which means it is locked by RTNL Jason
