Date:	Wed, 10 Jun 2015 15:14:18 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Marcelo Ricardo Leitner <mleitner@...hat.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Vlad Yasevich <vyasevich@...il.com>,
	Michio Honda <micchie@....wide.ad.jp>
Subject: Re: [PATCH v3 1/2] sctp: rcu-ify addr_waitq

On Wed, Jun 10, 2015 at 10:31:42AM -0300, Marcelo Ricardo Leitner wrote:
> On Tue, Jun 09, 2015 at 04:32:59PM -0300, Marcelo Ricardo Leitner wrote:
> > On Tue, Jun 09, 2015 at 07:36:38AM -0400, Neil Horman wrote:
> > > On Mon, Jun 08, 2015 at 05:37:05PM +0200, Hannes Frederic Sowa wrote:
> > > > On Mo, 2015-06-08 at 11:19 -0400, Neil Horman wrote:
> > > > > On Mon, Jun 08, 2015 at 04:59:18PM +0200, Hannes Frederic Sowa wrote:
> > > > > > On Mon, Jun 8, 2015, at 16:46, Hannes Frederic Sowa wrote:
> > > > > > > Hi Marcelo,
> > > > > > > 
> > > > > > > a few hints on rcuification, sorry I reviewed the code so late:
> > > > > > > 
> > > > > > > On Fri, Jun 5, 2015, at 19:08, mleitner@...hat.com wrote:
> > > > > > > > From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > > > > > 
> > > > > > > > That's needed for the next patch, so we break the lock 
> > > > > > > > inversion between
> > > > > > > > netns_sctp->addr_wq_lock and socket lock on
> > > > > > > > sctp_addr_wq_timeout_handler(). With this, we can traverse 
> > > > > > > > addr_waitq
> > > > > > > > without taking addr_wq_lock, taking it just for the write 
> > > > > > > > operations.
> > > > > > > > 
> > > > > > > > Signed-off-by: Marcelo Ricardo Leitner <
> > > > > > > > marcelo.leitner@...il.com>
> > > > > > > > ---
> > > > > > > > 
> > > > > > > > Notes:
> > > > > > > >     v2->v3:
> > > > > > > >       placed break statement on sctp_free_addr_wq_entry()
> > > > > > > >       removed unnecessary spin_lock noticed by Neil
> > > > > > > > 
> > > > > > > >  include/net/netns/sctp.h |  2 +-
> > > > > > > >  net/sctp/protocol.c      | 80
> > > > > > > >  +++++++++++++++++++++++++++++-------------------
> > > > > > > >  2 files changed, 49 insertions(+), 33 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/include/net/netns/sctp.h 
> > > > > > > > b/include/net/netns/sctp.h
> > > > > > > > index
> > > > > > > > 3573a81815ad9e0efb6ceb721eb066d3726419f0..9e53412c4ed829e8e4577
> > > > > > > > 7a6d95406d490dbaa75
> > > > > > > > 100644
> > > > > > > > --- a/include/net/netns/sctp.h
> > > > > > > > +++ b/include/net/netns/sctp.h
> > > > > > > > @@ -28,7 +28,7 @@ struct netns_sctp {
> > > > > > > >          * It is a list of sctp_sockaddr_entry.
> > > > > > > >          */
> > > > > > > >         struct list_head local_addr_list;
> > > > > > > > -       struct list_head addr_waitq;
> > > > > > > > +       struct list_head __rcu addr_waitq;
> > > > > > > >         struct timer_list addr_wq_timer;
> > > > > > > >         struct list_head auto_asconf_splist;
> > > > > > > >         spinlock_t addr_wq_lock;
> > > > > > > > diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
> > > > > > > > index
> > > > > > > > 53b7acde9aa37bf3d4029c459421564d5270f4c0..9954fb8c9a9455d5ad7a6
> > > > > > > > 27e2d7f9a1fef861fc2
> > > > > > > > 100644
> > > > > > > > --- a/net/sctp/protocol.c
> > > > > > > > +++ b/net/sctp/protocol.c
> > > > > > > > @@ -593,15 +593,47 @@ static void sctp_v4_ecn_capable(struct 
> > > > > > > > sock *sk)
> > > > > > > >         INET_ECN_xmit(sk);
> > > > > > > >  }
> > > > > > > >  
> > > > > > > > +static void sctp_free_addr_wq(struct net *net)
> > > > > > > > +{
> > > > > > > > +       struct sctp_sockaddr_entry *addrw;
> > > > > > > > +
> > > > > > > > +       spin_lock_bh(&net->sctp.addr_wq_lock);
> > > > > > > 
> > > > > > > Instead of holding spin_lock_bh you need to hold 
> > > > > > > rcu_read_lock_bh, so
> > > > > > > kfree_rcu does not call free function at once (in theory ;) ).
> > > > > > > 
> > > > > > > > +       del_timer(&net->sctp.addr_wq_timer);
> > > > > > > > +       list_for_each_entry_rcu(addrw, &net->sctp.addr_waitq, 
> > > > > > > > list) {
> > > > > > > > +               list_del_rcu(&addrw->list);
> > > > > > > > +               kfree_rcu(addrw, rcu);
> > > > > > > > +       }
> > > > > > > > +       spin_unlock_bh(&net->sctp.addr_wq_lock);
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +/* As there is no refcnt on sctp_sockaddr_entry, we must check 
> > > > > > > > inside
> > > > > > > > + * the lock if it wasn't removed from addr_waitq already, 
> > > > > > > > otherwise we
> > > > > > > > + * could double-free it.
> > > > > > > > + */
> > > > > > > > +static void sctp_free_addr_wq_entry(struct net *net,
> > > > > > > > +                                   struct sctp_sockaddr_entry 
> > > > > > > > *addrw)
> > > > > > > > +{
> > > > > > > > +       struct sctp_sockaddr_entry *temp;
> > > > > > > > +
> > > > > > > > +       spin_lock_bh(&net->sctp.addr_wq_lock);
> > > > > > > 
> > > > > > > I don't think this spin_lock operation is needed. The del_timer
> > > > > > > functions do synchronize themselves.
> > > > > > > 
> > > > > > 
> > > > > > Sorry, those above two locks are needed, they are not implied by 
> > > > > > other
> > > > > > locks.
> > > > > > 
> > > > > What makes you say that? Multiple contexts can issue mod_timer calls 
> > > > > on the
> > > > > same timer safely no, because of the internal locking?
> > > > 
> > > > That's true for timer handling but not to protect net->sctp.addr_waitq
> > > > list (Marcelo just explained it to me off-list). Looking at the patch
> > > > only in patchworks lost quite a lot of context you were already
> > > > discussing. ;)
> > > > 
> > > I can imagine :)
> > > 
> > > > We are currently checking if the double iteration can be avoided by
> > > > splicing addr_waitq on the local stack while holding the spin_lock and
> > > > later on notifying the sockets.
> > > > 
> > > As we discussed, this I think would make a good alternate approach.
> > 
> > I was experimenting on this but this would introduce another complex
> > logic instead, as not all elements are pruned from net->sctp.addr_waitq
> > at sctp_addr_wq_timeout_handler(), mainly ipv6 addresses in DAD state
> > (which I believe that break statement is misplaced and should be a
> > continue instead, I'll check on this later)
> > 
> > That means we would have to do the splice, process the loop, merge the
> > remaining elements with the new net->sctp.addr_waitq that was possibly
> > was built meanwhile and then squash oppositve events (logic currently in
> > sctp_addr_wq_mgmt() ), otherwise we could be issuing spurious events.
> > 
> > But it will probably do more harm than good as the double search will
> > usually hit the first list element on this 2nd search, unless the
> > element we are trying to remove was already removed from it (which is
> > rare, it's when user add and remove addresses too fast) or some other
> > address was skipped (DAD addresses).
> 
> Better thinking.. actually it may be the way to go. If we rcu-cify
> addr_waitq like that and if the user manage to add an address and remove
> it while the timeout handler is running, the system may emit just the
> address add and not the remove, while if we splice the list, this won't
> happen. 
> 
>   Marcelo
> 
> 
Thats a good point.  Seems like a list splice makes more sense here.
Neil

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