Re: [PATCH 2/2] fs: nfs: dir.c: Fix sparse error

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Dec 06, 2019 at 05:52:10PM +0000, Trond Myklebust wrote:
> Hi Paul,
> 
> On Fri, 2019-12-06 at 08:02 -0800, Paul E. McKenney wrote:
> > On Fri, Dec 06, 2019 at 08:46:40PM +0530, 
> > madhuparnabhowmik04@gmail.com wrote:
> > > From: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
> > > 
> > > This patch fixes the following errors:
> > > fs/nfs/dir.c:2353:14: error: incompatible types in comparison
> > > expression (different address spaces):
> > > fs/nfs/dir.c:2353:14:    struct list_head [noderef] <asn:4> *
> > > fs/nfs/dir.c:2353:14:    struct list_head *
> > > 
> > > caused due to directly accessing the prev pointer of
> > > a RCU protected list.
> > > Accessing the pointer using the macro list_prev_rcu() fixes this
> > > error.
> > > 
> > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
> > > ---
> > >  fs/nfs/dir.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > > index e180033e35cf..2035254cc283 100644
> > > --- a/fs/nfs/dir.c
> > > +++ b/fs/nfs/dir.c
> > > @@ -2350,7 +2350,7 @@ static int nfs_access_get_cached_rcu(struct
> > > inode *inode, const struct cred *cre
> > >  	rcu_read_lock();
> > >  	if (nfsi->cache_validity & NFS_INO_INVALID_ACCESS)
> > >  		goto out;
> > > -	lh = rcu_dereference(nfsi->access_cache_entry_lru.prev);
> > > +	lh = rcu_dereference(list_prev_rcu(&nfsi-
> > > >access_cache_entry_lru));
> > 
> > And as noted in the earlier email, what is preventing concurrent
> > insertions into  and deletions from this list?
> > 
> > o	This use of list_move_tail() is OK because it does not poison.
> > 	Though it isn't being all that friendly to lockless access to
> > 	->prev -- no WRITE_ONCE() in list_move_tail().
> > 
> > o	The use of list_add_tail() is not safe with RCU readers, though
> > 	they do at least partially compensate via use of smp_wmb()
> > 	in nfs_access_add_cache() before calling
> > nfs_access_add_rbtree().
> > 
> > o	The list_del() near the end of nfs_access_add_rbtree() will
> > 	poison the ->prev pointer.  I don't see how this is safe given
> > the
> > 	possibility of a concurrent call to
> > nfs_access_get_cached_rcu().
> 
> The pointer nfsi->access_cache_entry_lru is the head of the list, so it
> won't get poisoned. Furthermore, the objects it points to are freed
> using kfree_rcu(), so they will survive as long as we hold the rcu read
> lock. The object's cred pointers also points to something that is freed
> in an rcu-safe manner.
> 
> The problem here is rather that a racing list_del() can cause nfsi-
> >access_cache_entry_lru to be empty, which is presumably why Neil added
> that check plus the empty cred pointer check in the following line.
> 
> The barrier semantics may be suspect, although the spin unlock after
> list_del() should presumably guarantee release semantics?

Ah, OK, so you are only ever using ->prev only from the head of the list,
and presumably never do list_del() on the head itself.  (Don't laugh,
this does really happen as a way to remove the entire list, though
perhaps with list_del_init() rather than list_del().)

Maybe we should have a list_tail_rcu() that is only expected to work
on the head of the list?

							Thanx, Paul