> 2023年7月16日 01:19，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Sat, Jul 15, 2023 at 08:50:23AM +0800, Alan Huang wrote:
>>
>>> 2023年7月15日 07:23，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>
>>> On Tue, Jul 11, 2023 at 03:09:06PM +0000, Alan Huang wrote:
>>>> The objects are allocated with SLAB_TYPESAFE_BY_RCU, and there is
>>>> n->next = first within hlist_add_head_rcu() before rcu_assign_pointer(),
>>>> which modifies obj->obj_node.next. There may be readers holding the
>>>> reference of obj in lockless_lookup, and when updater modifies ->next,
>>>> readers can see the change immediately because ofSLAB_TYPESAFE_BY_RCU.
>>>>
>>>> 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>
>>>
>>> This is an interesting one!!!
>>>
>>> Now I am having a hard time believing that the smp_rmb() suffices.
>>>
>>>> ---
>>>> Changelog:
>>>> v1 -> v2: Use _ONCE to protect obj->key.
>>>>
>>>> Documentation/RCU/rculist_nulls.rst | 21 +++++++++++++--------
>>>> 1 file changed, 13 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst
>>>> index 21e40fcc08de..2a9f5a63d334 100644
>>>> --- a/Documentation/RCU/rculist_nulls.rst
>>>> +++ b/Documentation/RCU/rculist_nulls.rst
>>>> @@ -47,7 +47,7 @@ objects, which is having below type.
>>>> * reuse these object before the RCU grace period, we
>>>> * must check key after getting the reference on object
>>>> */
>>>> - if (obj->key != key) { // not the object we expected
>>>> + if (READ_ONCE(obj->key) != key) { // not the object we expected
>>>> put_ref(obj);
>>>> rcu_read_unlock();
>>>> goto begin;
>>>> @@ -64,10 +64,10 @@ but a version with an additional memory barrier (smp_rmb())
>>>> {
>>>> struct hlist_node *node, *next;
>>>> for (pos = rcu_dereference((head)->first);
>>>> - pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
>>>> + pos && ({ next = READ_ONCE(pos->next); smp_rmb(); prefetch(next); 1; }) &&
>>>
>>> Suppose that lockless_lookup() is delayed just before fetching pos->next,
>>> and that there were 17 more node to search in the list.
>>>
>>> Then consider the following sequence of events:
>>>
>>> o The updater deletes this same node and kmem_cache_free()s it.
>>>
>>> o Another updater kmem_cache_alloc()s that same memory and
>>> inserts it into an empty hash chain with a different key.
>>>
>>> o Then lockless_lookup() fetches pos->next and sees a NULL pointer,
>>> thus failing to search the remaining 17 nodes in the list,
>>> one of which had the desired key value.
>>>
>>> o The lookup algorithm resumes and sees the NULL return from
>>> lockless_lookup(), and ends up with a NULL obj.
>>>
>>> And this happens even with the strongest possible ordering
>>> everywhere.
>>>
>>> OK, yes, it is late on Friday. So what am I missing here?
>>
>> You missed nothing!
>>
>> The lockless_lockup should not be a function, but a macro like hlist_for_each_entry_rcu.
>
> How would you fix this using a macro?
With additional detection code. A moved object (in another chain) will have a different slot.
(I have sent patch v3. )
>
>>> Independent of that, does hlist_add_head_rcu() need to replace its
>>> "n->next = first" with "WRITE_ONCE(n->next, first)"?
>>
>> I think users who want to use hlist with SLAB_TYPESAFE_BY_RCU should use rculist_nulls?
>
> I believe that you are correct. Would you like to propose a patch, or
> would you rather I put something together? My current thought is to
Feel free to add.
One thing I think would be useful is to tell readers where the ‘next' is.
The document mentions ’next’ many times, but it’s hard for me, as a reader, to realize that
the ‘next' is within hlist_add_head_rcu(). (I have no idea where to put the hint.)
> keep the examples, but to show why the one with smp_rmb() is broken.
I think the example needs to be fixed. :)
>
>> I didn’t find a case using hlist with SLAB_TYPESAFE_BY_RCU, but I did find a case using list
>> with SLAB_TYPESAFE_BY_RCU in drivers/gpu/drm/i915, the driver also doesn’t use _ONCE
>> on the fields of the objects allocated with SLAB_TYPESAFE_BY_RCU.
>
> Feel free to send them a patch, though I cannot speak for their
> reception of it.
>
> Thanx, Paul
>
>>> Thanx, Paul
>>>
>>>> ({ obj = hlist_entry(pos, typeof(*obj), obj_node); 1; });
>>>> pos = rcu_dereference(next))
>>>> - if (obj->key == key)
>>>> + if (READ_ONCE(obj->key) == key)
>>>> return obj;
>>>> return NULL;
>>>> }
>>>> @@ -111,8 +111,13 @@ 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
>>>> + WRITE_ONCE(obj->key, key);
>>>> + /*
>>>> + * We need to make sure obj->key is updated before obj->obj_node.next
>>>> + * and obj->refcnt.
>>>> + */
>>>> + smp_wmb();
>>>> + atomic_set(&obj->refcnt, 1);
>>>> hlist_add_head_rcu(&obj->obj_node, list);
>>>> unlock_chain(); // typically a spin_unlock()
>>>>
>>>> @@ -165,12 +170,12 @@ Note that using hlist_nulls means the type of 'obj_node' field of
>>>> begin:
>>>> rcu_read_lock();
>>>> hlist_nulls_for_each_entry_rcu(obj, node, head, obj_node) {
>>>> - if (obj->key == key) {
>>>> + if (READ_ONCE(obj->key) == key) {
>>>> if (!try_get_ref(obj)) { // might fail for free objects
>>>> rcu_read_unlock();
>>>> goto begin;
>>>> }
>>>> - if (obj->key != key) { // not the object we expected
>>>> + if (READ_ONCE(obj->key) != key) { // not the object we expected
>>>> put_ref(obj);
>>>> rcu_read_unlock();
>>>> goto begin;
>>>> @@ -206,7 +211,7 @@ hlist_add_head_rcu().
>>>> */
>>>> obj = kmem_cache_alloc(cachep);
>>>> lock_chain(); // typically a spin_lock()
>>>> - obj->key = key;
>>>> + WRITE_ONCE(obj->key, key);
>>>> atomic_set_release(&obj->refcnt, 1); // key before refcnt
>>>> /*
>>>> * insert obj in RCU way (readers might be traversing chain)
>>>> --
>>>> 2.34.1
