Re: [PATCH v3 1/2] sctp: rcu-ify addr_waitq

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

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@redhat.com wrote:
> > From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.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@gmail.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..9e53412c4ed829e8e45777a6d95406d490dbaa75
> > 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..9954fb8c9a9455d5ad7a627e2d7f9a1fef861fc2
> > 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.

Bye,
Hannes