Paul E. McKenney wrote: > On Wed, Aug 17, 2005 at 09:25:52AM +0100, Steven Whitehouse wrote: > >>Hi, >> >>On Tue, Aug 16, 2005 at 07:01:57PM -0700, Paul E. McKenney wrote: >> >>>On Tue, Aug 16, 2005 at 05:09:29PM -0700, Suzanne Wood wrote: >>>[ . . . ] >>> >>>>A read-side critical section is marked to protect the dereference of the >>>>dn_ptr and assignment to dn_db which is a pointer to a dn_dev. (struct >>>>net_device is defined in /linux/netdevice.h and its dn_ptr in >>>>/include/net/dn_dev.h) Should this rcu-protection be extended to the line >>>>following rcu_read_lock()? Even though use_long is a simple char, it >>>>appears to be a member of an rcu-protected structure. >>> >>>Looks to me that this could indeed be a problem -- the structure >>>pointed to by dn_db could potentially be freed immediately after the >>>rcu_read_unlock(), unless there is some other non-obvious locking >>>mechanism protecting it. In which case, why the rcu_read_lock() >>>and rcu_read_unlock()... >>> >>> Thanx, Paul >> >>The dev->dn_ptr points to the DECnet specific portion of a net device which >>is allocated in dn_dev.c/dn_dev_up and freed in dn_dev.c/dn_dev_delete when >>the net device goes up and down. >> >>So I think you are right in that as far as I can see, its possible for a >>net device going down to race with this, but the window of opportunity is >>very small indeed (in fact possibly zero?) due to the ordering of operations >>in dn_dev_delete where dev->dn_ptr is set to NULL (esentially preventing >>any more DECnet packets being received on that device) before flushing all >>neighbours and only then releasing dn_db. > > > I agree that the window is quite small, but suppose that there was a > lengthy interrupt received just after the rcu_read_unlock()? > > >>Also, Patrick Caulfield is maintaining this code now, so I've added him to >>the CC list. Thanks for the report though, > > > How about the following patch? Untested, but seems pretty straightforward. > > Thanx, Paul > > Fix RCU race condition in dn_neigh_construct(). > > --- > > Signed-off-by: <[EMAIL PROTECTED]> > > diff -urpNa -X dontdiff linux-2.6.13-rc6/net/decnet/dn_neigh.c > linux-2.6.13-rc6-db_db/net/decnet/dn_neigh.c > --- linux-2.6.13-rc6/net/decnet/dn_neigh.c 2005-08-08 19:59:25.000000000 > -0700 > +++ linux-2.6.13-rc6-db_db/net/decnet/dn_neigh.c 2005-08-17 > 07:08:10.000000000 -0700 > @@ -148,12 +148,12 @@ static int dn_neigh_construct(struct nei > > __neigh_parms_put(neigh->parms); > neigh->parms = neigh_parms_clone(parms); > - rcu_read_unlock(); > > if (dn_db->use_long) > neigh->ops = &dn_long_ops; > else > neigh->ops = &dn_short_ops; > + rcu_read_unlock(); > > if (dn->flags & DN_NDFLAG_P3) > neigh->ops = &dn_phase3_ops; > >
Looks fine to me. I've done a quick test and it doesn't seem to interfere - not that I expected it to :) Thanks. -- patrick - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
