> 2023年7月22日 上午6:49，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Wed, Jul 19, 2023 at 12:08:33AM +0800, Alan Huang wrote:
>>
>>> 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?
>
> You are asking about the is_a_nulls() value? If so, it works on both
> 32-bit and 64-bit machines. They each have enough bits for the nulls
> value to cover all possible two-byte objects in the full address space.
>
> If that wasn't what you were asking, please help me with your question.
I’m asking the ‘back-pointer’, which
"Looks similar to the is_a_nulls() pointer, but in each element instead of
just at the end”
If I understand correctly, we need to store the nulls in the upper unused bits.
And we only have several bits unused. One advantage I can think of is that it
will improve the performance once we have something like the Intel Upper Address Ignore (UAI),
which also works only for 64-bit machine.
>
> Thanx, Paul
>
>>> 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
