> 2023年7月18日 03:06，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Tue, Jul 18, 2023 at 01:53:10AM +0800, Alan Huang wrote:
>>> 2023年7月18日 00:02，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>> On Sun, Jul 16, 2023 at 07:21:28PM +0800, Alan Huang wrote:
>>>>> 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. :)
>>>
>>> Even better! I will take a look, but in the meantime, would you be
>>> interested in updating the wording to explain how the back-pointer works?
>>
>> Which document needs to be updated?
>> And is there anything that I can refer to? It’s the first time I have ever heard about it.
>
> Documentation/RCU/rculist_nulls.rst, the one that you are updating.
>
> There admittedly isn't a whole lot of commentary.
>
>>> (Looks similar to the is_a_nulls() pointer, but in each element instead of
>>> just at the end. One advantage is the ability to detect a move mid-list,
>>> though that is not a big deal in well-tuned hash tables, which tend to
>>> have short hash chains. The need to move elements to the front of the
>>> destination list remains, though in both cases only if it has been less
>>> than a grace period since the last move.)
>>
>> Looks like that I need to learn it first. :)
>
> Well, you wrote the code, so... ;-)
If I understand correctly, it works only for 64-bit machines?
And the number of slots of the hash table will be limited?
>
> Thanx, Paul
>
>>> Thanx, Paul
>>>
>>>>>> 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
