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.
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
>
>
