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

[Roman Gushchin]

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 <klamm@xxxxxxxxxxxxxx>
> > Reported-by: Boris Zhmurov <zhmurov@xxxxxxxxxxxxxx>
> > ---
> >   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?

Regards,
Roman

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