> 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. 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.
