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

[Peter Zijlstra]

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().

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