netdev - Re: [PATCH net 2/3] sctp: hold transport before we access t->asoc in sctp proc

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Message-ID: <1453406236.1223.381.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 21 Jan 2016 11:57:16 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:	Xin Long <lucien.xin@...il.com>,
	network dev <netdev@...r.kernel.org>,
	linux-sctp@...r.kernel.org, Vlad Yasevich <vyasevich@...il.com>,
	daniel@...earbox.net, davem@...emloft.net
Subject: Re: [PATCH net 2/3] sctp: hold transport before we access t->asoc
 in sctp proc

On Thu, 2016-01-21 at 17:37 -0200, Marcelo Ricardo Leitner wrote:
> On Thu, Jan 21, 2016 at 11:27:36AM -0800, Eric Dumazet wrote:
> > On Fri, 2016-01-22 at 01:49 +0800, Xin Long wrote:
> > > Previously, before rhashtable, /proc assoc listing was done by
> > > read-locking the entire hash entry and dumping all assocs at once, so we
> > > were sure that the assoc wasn't freed because it wouldn't be possible to
> > > remove it from the hash meanwhile.
> > > 
> > > Now we use rhashtable to list transports, and dump entries one by one.
> > > That is, now we have to check if the assoc is still a good one, as the
> > > transport we got may be being freed.
> > > 
> > > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > > ---
> > >  net/sctp/proc.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/net/sctp/proc.c b/net/sctp/proc.c
> > > index 684c5b3..c74a810 100644
> > > --- a/net/sctp/proc.c
> > > +++ b/net/sctp/proc.c
> > > @@ -380,6 +380,8 @@ static int sctp_assocs_seq_show(struct seq_file *seq, void *v)
> > >  	}
> > >  
> > >  	transport = (struct sctp_transport *)v;
> > 
> > What protects you from this structure already being freed ?
> 
> rcu, rhashtable_walk_start() at sctp_assocs_seq_start() starts an
> (implicit from this POV) rcu_read_lock() for us which is unlocked only
> when the walking is terminated, thus covering this _show.
> 
> > > +	if (!sctp_transport_hold(transport))
> > > +		return 0;
> > 
> > If this is rcu, then you do not need to increment the refcount, and
> > decrement it later.
> 
> It's an implicit hold on sctp asoc.
> 
> This code is using contents from asoc pointer, which is not proctected
> by rcu. As transport has a hold on the asoc, it's enough to just hold
> the transport and not the asoc too, as we had to do in the previous
> patch.

Then it means fast path also need to do this sctp_transport_hold() ?

If sctp_association_put() was called from sctp_transport_destroy_rcu()
(ie after rcu grace period), you would not need to increment/decrement
the transport refcount.

Normally, RCU protection does not need to change the refcount, unless we
need to keep an object alive after escaping the rcu section.