On Sun, Jun 18, 2023 at 01:55:10AM +0800, Alan Huang wrote: > > > 2023年6月18日 01:23，Matthew Wilcox <willy@xxxxxxxxxxxxx> 写道： > > > > 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 > > > > "at the same time" doesn't make a lot of sense to me. CPUs don't do > > anything "at the same time". I think the way this is worded now is > > fine; I tried coming up with a few variants of this, but none are as > > clear and succinct as what is there now. > > > >> 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); > > > > Perhaps this could be a little clearer for those of us who aren't as > > deep into the memory model as others ... are you saying that the > > atomic_set_release() would only order references to obj->refcount and > > would not order accesses to obj->key? Because reading this: > > > > The use of ACQUIRE and RELEASE operations generally precludes the need > > for other sorts of memory barrier. In addition, a RELEASE+ACQUIRE pair is > > -not- guaranteed to act as a full memory barrier. However, after an > > ACQUIRE on a given variable, all memory accesses preceding any prior > > RELEASE on that same variable are guaranteed to be visible. In other > > words, within a given variable's critical section, all accesses of all > > previous critical sections for that variable are guaranteed to have > > completed. > > > > makes me think that this example is fine; anybody doing a load-acquire > > on obj->refcount will see the update to obj->key that happened before > > the store-release to obj->refcount. > > Two memory ordering required here, atomic_set_release only provides one of them (the one you mentioned) > > The objects are allocated with SLAB_TYPESAFE_BY_RCU, and there is: > > n->next = first; > > in hlist_add_head_rcu, which modifies obj->obj_node.next. But isn't that the bug? ie this assignment should be WRITE_ONCE() or smp_store_release() or rcu_assign_pointer() or whichever of these similar functions is actually appropriate? > So, we must make sure obj->key is updated before obj->obj_node.next, without smp_wmb(), we can read > the new 'obj->obj_node.next’ value and previous value of 'obj->key’ at the same time, and in this case, we > can not detect the movement of the object. > > The following link might be helpful: > > https://patchwork.ozlabs.org/project/netdev/patch/491C282A.5050802@xxxxxxxxxxxxx/ > > > > > > I am not an expert and would welcome the opportunity to learn more here. > >
