Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t

[Will Deacon]

On Thu, Nov 02, 2017 at 04:21:56PM -0400, Alan Stern wrote:
> I was trying to think of something completely different.  If you have a
> release/acquire to the same address, it creates a happens-before
> ordering:
> 
> 	Access x
> 	Release a
> 	Acquire a
> 	Access y
> 
> Here is the access to x happens-before the access to y.  This is true
> even on x86, even in the presence of forwarding -- the CPU still has to
> execute the instructions in order.  But if the release and acquire are
> to different addresses:
> 
> 	Access x
> 	Release a
> 	Acquire b
> 	Access y
> 
> then there is no happens-before ordering for x and y -- the CPU can
> execute the last two instructions before the first two.  x86 and
> PowerPC won't do this, but I believe ARMv8 can.  (Please correct me if
> it can't.)

Release/Acquire are RCsc on ARMv8, so they are ordered irrespective of
address.

Will