Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls

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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? ;-)

But incremental progress and all, so this LGTM:
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

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 {




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