> 2023年6月10日 13:37，Alan Huang <mmpgouride@xxxxxxxxx> 写道： > > Hi Paul, > >> 2023年6月10日 07:42，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道： >> >> On Fri, Jun 09, 2023 at 07:12:06PM +0000, SeongJae Park wrote: >>> On Fri, 19 May 2023 14:52:50 -0400 Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: >>> >>>> On Thu, May 18, 2023 at 6:40 PM SeongJae Park <sj@xxxxxxxxxx> wrote: >>>>> >>>>> The document says we can avoid extra smp_rmb() in lockless_lookup() and >>>>> extra _release() in insert function when hlist_nulls is used. However, >>>>> the example code snippet for the insert function is still using the >>>>> extra _release(). Drop it. >>>>> >>>>> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx> >>>>> --- >>>>> Documentation/RCU/rculist_nulls.rst | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst >>>>> index 5cd6f3f8810f..463270273d89 100644 >>>>> --- a/Documentation/RCU/rculist_nulls.rst >>>>> +++ b/Documentation/RCU/rculist_nulls.rst >>>>> @@ -191,7 +191,7 @@ scan the list again without harm. >>>>> obj = kmem_cache_alloc(cachep); >>>>> lock_chain(); // typically a spin_lock() >>>>> obj->key = key; >>>>> - atomic_set_release(&obj->refcnt, 1); // key before refcnt >>>>> + atomic_set(&obj->refcnt, 1); >>>>> /* >>>>> * insert obj in RCU way (readers might be traversing chain) >>>>> */ >>>> >>>> If write to ->refcnt of 1 is reordered with setting of ->key, what >>>> prevents the 'lookup algorithm' from doing a key match (obj->key == >>>> key) before the refcount has been initialized? >>>> >>>> Are we sure the reordering mentioned in the document is the same as >>>> the reordering prevented by the atomic_set_release()? >>> >>> Paul, may I ask your opinion? >> >> The next line of code is this: >> >> hlist_nulls_add_head_rcu(&obj->obj_node, list); >> >> If I understand the code correctly, obj (and thus *obj) are not >> visible to readers before the hlist_nulls_add_head_rcu(). And >> hlist_nulls_add_head_rcu() uses rcu_assign_pointer() to ensure that >> initialization (including both ->key and ->refcnt) is ordered before >> list insertion. >> >> Except that this memory is being allocated from a slab cache that was >> created with SLAB_TYPESAFE_BY_RCU. This means that there can be readers >> who gained a reference before this object was freed, and who still hold >> their references. >> >> Unfortunately, the implementation of try_get_ref() is not shown. However, >> if ->refcnt is non-zero, this can succeed, and if it succeeds, we need >> the subsequent check of obj->key with key in the lookup algorithm to >> be stable. For this check to be stable, try_get_ref() needs to use an >> atomic operation with at least acquire semantics (kref_get_unless_zero() >> would work), and this must pair with something in the initialization. >> >> So I don't see how it is safe to weaken that atomic_set_release() to >> atomic_set(), even on x86. > > I totally agree, but only in the case of using hlist_nulls. > > That means, atomic_set_release() is not enough in the case without using hlist_nulls, > we must ensure that storing to obj->next (in hlist_add_head_rcu) is ordered before storing Typo: not before, but after. > to obj->key. Otherwise, we can get the new ‘next' and the old ‘key' in which case we can’t detect > an object movement(from one chain to another). > > So, I’m afraid that the atomic_set_release() in insertion algorithm without using hlist_nulls should > change back to: > > smp_wmb(); > atomic_set(&obj->refcnt, 1); > > Thanks, > Alan > >> >> Or am I missing something subtle here? >> >> Thanx, Paul >> >>> Thanks, >>> SJ >>> >>>> >>>> For the other 3 patches, feel free to add: >>>> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> >>>> >>>> thanks, >>>> >>>> - Joel
