On Tue, Jul 18, 2023 at 10:48:07PM +0800, Alan Huang wrote:
>
> > 2023年7月18日 21:49，Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 写道：
> >
> > On 7/17/23 14:03, Paul E. McKenney wrote:
> >> From: Alan Huang <mmpgouride@xxxxxxxxx>
> >> When the objects managed by rculist_nulls are allocated with
> >> SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an object
> >> even though it is just now being added, which means the modification of
> >> ->next is visible to readers. This patch therefore uses WRITE_ONCE()
> >> for assignments to ->next.
> >> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
> >> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >
> > Did we ever conclude that the READ_ONCE() counterparts were not needed? ;-)
>
> Read-side is already protected by rcu_dereference_raw() in hlist_nulls_for_each_entry_{rcu, safe}.
It turns out that different traversal synchronization designs want
different pointers using WRITE_ONCE().
> > But incremental progress and all, so this LGTM:
> > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
I will apply all four on my next rebase, thank you!
Thanx, Paul
> > thanks,
> >
> > - Joel
> >
> >
> >> ---
> >> include/linux/rculist_nulls.h | 4 ++--
> >> 1 file changed, 2 insertions(+), 2 deletions(-)
> >> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> >> index ba4c00dd8005..89186c499dd4 100644
> >> --- a/include/linux/rculist_nulls.h
> >> +++ b/include/linux/rculist_nulls.h
> >> @@ -101,7 +101,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
> >> {
> >> struct hlist_nulls_node *first = h->first;
> >> - n->next = first;
> >> + WRITE_ONCE(n->next, first);
> >> WRITE_ONCE(n->pprev, &h->first);
> >> rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
> >> if (!is_a_nulls(first))
> >> @@ -137,7 +137,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
> >> last = i;
> >> if (last) {
> >> - n->next = last->next;
> >> + WRITE_ONCE(n->next, last->next);
> >> n->pprev = &last->next;
> >> rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
> >> } else {
> >
>
