linux-kernel - Re: [PATCH v2 tip/core/rcu 05/13] decnet: Apply rcu_access

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Message-ID: <20131009224604.GF5790@linux.vnet.ibm.com>
Date:	Wed, 9 Oct 2013 15:46:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...g.ch>, Gao feng <gaofeng@...fujitsu.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	linux-decnet-user@...ts.sourceforge.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 05/13] decnet: Apply rcu_access_pointer()
 to avoid sparse false positive

On Wed, Oct 09, 2013 at 03:28:42PM -0700, Josh Triplett wrote:
> On Wed, Oct 09, 2013 at 02:29:38PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > The sparse checking for rcu_assign_pointer() was recently upgraded
> > to reject non-__kernel address spaces.  This also rejects __rcu,
> > which is almost always the right thing to do.  However, the use in
> > dn_insert_route() is legitimate: It is assigning a pointer to an element
> 
> Nit: "uses ... are", not "use ... is". :)

Don't I already have "use ... is"?

							Thanx, Paul

> > from an RCU-protected list, and all elements of this list are already
> > visible to caller.
> > 
> > This commit therefore silences this false positive by laundering the
> > pointer using rcu_access_pointer() as suggested by Josh Triplett.
> > 
> > Reported-by: kbuild test robot <fengguang.wu@...el.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Thomas Graf <tgraf@...g.ch>
> > Cc: Gao feng <gaofeng@...fujitsu.com>
> > Cc: Stephen Hemminger <shemminger@...tta.com>
> > Cc: linux-decnet-user@...ts.sourceforge.net
> > Cc: netdev@...r.kernel.org
> 
> With or without the above typo fix:
> 
> Reviewed-by: Josh Triplett <josh@...htriplett.org>
> 
> > ---
> >  net/decnet/dn_route.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/decnet/dn_route.c b/net/decnet/dn_route.c
> > index fe32388ea24f..3b1357bcfc92 100644
> > --- a/net/decnet/dn_route.c
> > +++ b/net/decnet/dn_route.c
> > @@ -345,7 +345,7 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou
> >  			/* Put it first */
> >  			*rthp = rth->dst.dn_next;
> >  			rcu_assign_pointer(rth->dst.dn_next,
> > -					   dn_rt_hash_table[hash].chain);
> > +					   rcu_access_pointer(dn_rt_hash_table[hash].chain));
> >  			rcu_assign_pointer(dn_rt_hash_table[hash].chain, rth);
> >  
> >  			dst_use(&rth->dst, now);
> > @@ -358,7 +358,8 @@ static int dn_insert_route(struct dn_route *rt, unsigned int hash, struct dn_rou
> >  		rthp = &rth->dst.dn_next;
> >  	}
> >  
> > -	rcu_assign_pointer(rt->dst.dn_next, dn_rt_hash_table[hash].chain);
> > +	rcu_assign_pointer(rt->dst.dn_next,
> > +			   rcu_access_pointer(dn_rt_hash_table[hash].chain));
> >  	rcu_assign_pointer(dn_rt_hash_table[hash].chain, rt);
> >  
> >  	dst_use(&rt->dst, now);
> > -- 
> > 1.8.1.5
> > 
> 

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