Subject: Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 21 May 2013 05:58:56 -0700

On Tue, May 21, 2013 at 04:46:54PM +0400, Roman Gushchin wrote:
> On 21.05.2013 16:09, Paul E. McKenney wrote:
> >On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote:
> >>Hi, all!
> >>
> >>This is a fix for a problem described here:
> >>https://lkml.org/lkml/2013/4/16/371 .
> >>---
> >>
> >>Some network functions (udp4_lib_lookup2(), for instance) use the
> >>hlist_nulls_for_each_entry_rcu macro in a way that assumes restarting
> >>of a loop. In this case, it is strictly necessary to reread the head->first
> >>value from the memory before each scan.
> >>Without additional hints, gcc caches this value in a register. In this case,
> >>if a cached node is moved to another chain during the scan, we can loop
> >>forever getting wrong nulls values and restarting the loop uninterruptedly.
> >>
> >>Signed-off-by: Roman Gushchin <[email-masked]>
> >>Reported-by: Boris Zhmurov <[email-masked]>
> >>---
> >>  include/linux/rculist_nulls.h | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> >>index 2ae1371..efd51bf 100644
> >>--- a/include/linux/rculist_nulls.h
> >>+++ b/include/linux/rculist_nulls.h
> >>@@ -37,8 +37,9 @@ static inline void hlist_nulls_del_init_rcu(struct
> >>hlist_nulls_node *n)
> >>  	}
> >>  }
> >>
> >>-#define hlist_nulls_first_rcu(head) \
> >>-	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
> >>+#define hlist_nulls_first_rcu(head)			\
> >>+	(*((struct hlist_nulls_node __rcu __force **)	\
> >>+	   &((volatile typeof(*head) *)head)->first))
> >
> >Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here?
> 
> It will be nice, but will require to keep the old variant too (for
> using in hlist_nulls_add_head_rcu() as in rcu_assign_pointer()
> argument). Do you think, it's better?

Both ACCESS_ONCE() and rcu_dereference_raw() can be used by updaters
as well as readers, so yes, I do think that it is better.  Better to
keep the encapsulation rather than having to search for lots of volatile
casts should this idiom ever need to change.

							Thanx, Paul