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... ;-)
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
>
>
