Re: [PATCH v2] docs/RCU: Bring smp_wmb() back

[Alan Huang <mmpgouride@gmail.com>]

> 2023年6月18日 01:23，Matthew Wilcox <willy@infradead.org> 写道：
> 
> On Sat, Jun 17, 2023 at 02:53:46PM +0000, Alan Huang wrote:
>> 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@gmail.com>
>> ---
>> Documentation/RCU/rculist_nulls.rst | 9 +++++++--
>> 1 file changed, 7 insertions(+), 2 deletions(-)
>> 
>> diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst
>> index e06ed40bb6..77244adbdf 100644
>> --- a/Documentation/RCU/rculist_nulls.rst
>> +++ b/Documentation/RCU/rculist_nulls.rst
>> @@ -98,7 +98,7 @@ Quoting Corey Minyard::
>> ----------------------
>> 
>> We need to make sure a reader cannot read the new 'obj->obj_node.next' value
>> -and previous value of 'obj->key'. Otherwise, an item could be deleted
>> +and previous value of 'obj->key' at the same time. Otherwise, an item could be deleted
> 
> "at the same time" doesn't make a lot of sense to me.  CPUs don't do
> anything "at the same time".  I think the way this is worded now is
> fine; I tried coming up with a few variants of this, but none are as
> clear and succinct as what is there now.
> 
>> from a chain, and inserted into another chain. If new chain was empty
>> before the move, 'next' pointer is NULL, and lockless reader can not
>> detect the fact that it missed following items in original chain.
>> @@ -112,7 +112,12 @@ 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
>> +  /*
>> +  * we need to make sure obj->key is updated before obj->obj_node.next
>> +  * and obj->refcnt
>> +  */
>> +  smp_wmb();
>> +  atomic_set(&obj->refcnt, 1);
> 
> Perhaps this could be a little clearer for those of us who aren't as
> deep into the memory model as others ... are you saying that the
> atomic_set_release() would only order references to obj->refcount and
> would not order accesses to obj->key?  Because reading this:
> 
>     The use of ACQUIRE and RELEASE operations generally precludes the need
>     for other sorts of memory barrier.  In addition, a RELEASE+ACQUIRE pair is
>     -not- guaranteed to act as a full memory barrier.  However, after an
>     ACQUIRE on a given variable, all memory accesses preceding any prior
>     RELEASE on that same variable are guaranteed to be visible.  In other
>     words, within a given variable's critical section, all accesses of all
>     previous critical sections for that variable are guaranteed to have
>     completed.
> 
> makes me think that this example is fine; anybody doing a load-acquire
> on obj->refcount will see the update to obj->key that happened before
> the store-release to obj->refcount.

Two memory ordering required here, atomic_set_release only provides one of them (the one you mentioned)

The objects are allocated with SLAB_TYPESAFE_BY_RCU, and there is:
	
		n->next = first;

in hlist_add_head_rcu, which modifies obj->obj_node.next.

So, we must make sure obj->key is updated before obj->obj_node.next, without smp_wmb(), we can read
the new 'obj->obj_node.next’ value and previous value of 'obj->key’ at the same time, and in this case, we
can not detect the movement of the object.

The following link might be helpful:

		https://patchwork.ozlabs.org/project/netdev/patch/491C282A.5050802@cosmosbay.com/


> 
> I am not an expert and would welcome the opportunity to learn more here.