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Message-ID: <20150609113638.GA27870@hmsreliant.think-freely.org>
Date: Tue, 9 Jun 2015 07:36:38 -0400
From: Neil Horman <nhorman@...driver.com>
To: Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc: mleitner@...hat.com, 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 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.
Neil
> Bye,
> Hannes
>
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