Re: [PATCH 4/4] Docs/RCU/rculist_nulls: Drop unnecessary '_release' in insert function

[SeongJae Park <sj@kernel.org>]

On Fri, 9 Jun 2023 16:42:59 -0700 "Paul E. McKenney" <paulmck@kernel.org> wrote:

> 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@joelfernandes.org> wrote:
> > 
> > > On Thu, May 18, 2023 at 6:40 PM SeongJae Park <sj@kernel.org> 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@kernel.org>
> > > > ---
> > > >  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.

Thank you for the nice explanation, and I agree.

> 
> Or am I missing something subtle here?

I found the text is saying extra _release() in insert function is not
needed[1], and I thought it means the atomic_set_release().  Am I misreading
it?  If not, would it be better to fix the text, for example, like below?

```
--- a/Documentation/RCU/rculist_nulls.rst
+++ b/Documentation/RCU/rculist_nulls.rst
@@ -129,8 +129,7 @@ very very fast (before the end of RCU grace period)
 Avoiding extra smp_rmb()
 ========================

-With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
-and extra _release() in insert function.
+With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup().

 For example, if we choose to store the slot number as the 'nulls'
 end-of-list marker for each slot of the hash table, we can detect
@@ -182,6 +181,9 @@ scan the list again without harm.
 2) Insert algorithm
 -------------------

+Same to the above one, but uses hlist_nulls_add_head_rcu() instead of
+hlist_add_head_rcu().
+
 ::

   /*
@@ -191,7 +193,7 @@ 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)
    */
```

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/RCU/rculist_nulls.rst#n133


Thanks,
SJ

> 
> 							Thanx, Paul
> 
> > Thanks,
> > SJ
> > 
> > > 
> > > For the other 3 patches, feel free to add:
> > > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > 
> > > thanks,
> > > 
> > >  - Joel