On Sat, Jun 17, 2023 at 02:53:46PM +0000, Alan Huang wrote: > There are two memory ordering required in the insertion algorithm, > we need to make sure obj->key is updated before obj->obj_node.next > and obj->refcnt, atomic_set_release is not enough to provide the > required memory barrier. > > Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx> > --- > Documentation/RCU/rculist_nulls.rst | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst > index e06ed40bb6..77244adbdf 100644 > --- a/Documentation/RCU/rculist_nulls.rst > +++ b/Documentation/RCU/rculist_nulls.rst > @@ -98,7 +98,7 @@ Quoting Corey Minyard:: > ---------------------- > > We need to make sure a reader cannot read the new 'obj->obj_node.next' value > -and previous value of 'obj->key'. Otherwise, an item could be deleted > +and previous value of 'obj->key' at the same time. Otherwise, an item could be deleted > from a chain, and inserted into another chain. If new chain was empty > before the move, 'next' pointer is NULL, and lockless reader can not > detect the fact that it missed following items in original chain. > @@ -112,7 +112,12 @@ detect the fact that it missed following items in original chain. > obj = kmem_cache_alloc(...); > lock_chain(); // typically a spin_lock() > obj->key = key; > - atomic_set_release(&obj->refcnt, 1); // key before refcnt > + /* > + * we need to make sure obj->key is updated before obj->obj_node.next > + * and obj->refcnt > + */ > + smp_wmb(); > + atomic_set(&obj->refcnt, 1); You lost me on this one. >From what I can see, in the original the atomic_set_release() provided ordering between the ->key assignment and the ->refcnt assignment, and then the rcu_assign_pointer() within hlist_add_head_rcu() provided ordering between both the ->key and the ->refcnt assignments on the one hand and the list insertion on the other. What am I missing here? Thanx, Paul > hlist_add_head_rcu(&obj->obj_node, list); > unlock_chain(); // typically a spin_unlock() > > -- > 2.34.1 >
