Re: [RFC][PATCH 7/7] kref: Implement using refcount_t

[Boqun Feng]

On Tue, Nov 15, 2016 at 02:01:54PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 15, 2016 at 08:33:37PM +0800, Boqun Feng wrote:
> > Hi Peter,
> > 
> > On Mon, Nov 14, 2016 at 06:39:53PM +0100, Peter Zijlstra wrote:
> > [...]
> > > +/*
> > > + * Similar to atomic_dec_and_test(), it will BUG on underflow and fail to
> > > + * decrement when saturated at UINT_MAX.
> > > + *
> > > + * Provides release memory ordering, such that prior loads and stores are done
> > > + * before a subsequent free.
> > 
> > I'm not sure this is correct, the RELEASE semantics is for the STORE
> > part of cmpxchg, and semantically it will guarantee that memory
> > operations after cmpxchg won't be reordered upwards, for example, on
> > ARM64, the following code:
> > 
> > 	WRITE_ONCE(x, 1)
> > 	
> > 	atomic_cmpxchg_release(&a, 1, 2);
> > 	  r1 = ll(&a)
> > 	  if (r1 == 1) {
> > 	    sc_release(&a, 2);
> > 	  }
> > 	
> > 	free()
> > 
> > could be reordered as, I think:
> > 
> > 	atomic_cmpxchg_release(&a, 1, 2);
> > 	  r1 = ll(&a)
> > 	  if (r1 == 1) {
> > 	    free()
> > 	    WRITE_ONCE(x, 1)
> > 	    sc_release(&a, 2);
> > 	  }
> > 
> > Of course, we need to wait for Will to confirm about this. But if this
> > could happen, we'd better to use a smp_mb()+atomic_cmpxchg_relaxed()
> > here and for other refcount_dec_and_*().
> 
> Can't happen I think because of the control dependency between
> dec_and_test() and free().
> 
> That is, the cmpxchg_release() must complete to determine if it was
> successful or it needs a retry. The success, combined with the state of
> the variable will then determine if we call free().
> 

The thing is that determination of the variable's state(i.e.
store_release() succeeds) and the actual writeback to memory are two
separate events. So yes, free() won't execute before store_release()
commits successfully, but there is no barrier here to order the memory
effects of store_release() and free().

See a similar example:

https://marc.info/?l=linux-s390&m=146604339321723&w=2


But as I said, we actually only need the pairing of orderings:

1) load part of cmpxchg -> free() 
2) object accesses -> store part of cmpxchg

Ordering #1 can be achieved via control dependency as you pointed out
that free()s very much includes stores. And ordering #2 can be achieved
with RELEASE.

So the code is right, I just thought the comment may be misleading. The
reason we use cmpxchg_release() is just for achieving ordering #2, and
not to order "prior loads and stores" with "a subsequent free".

Am I missing some subtle orderings here?

Regards,
Boqun

> So I don't think we can get free() (which very much includes stores) to
> happen before the store-release.

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