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Date: Wed, 15 Nov 2017 21:03:07 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will.deacon@arm.com>,
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Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in
 atomic_t
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References: <20171115180540.GQ19071@arm.com>
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On Wed, Nov 15, 2017 at 02:15:19PM -0500, Alan Stern wrote:
> On Wed, 15 Nov 2017, Will Deacon wrote:
> 
> > 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.
> 
> Ah, okay, thanks.
> 
> In any case, we have considered removing this ordering constraint
> (store-release followed by load-acquire for the same location) from the
> Linux-kernel memory model.

Why? Its a perfectly sensible construct.

> I'm not aware of any code in the kernel that depends on it.  Do any of
> you happen to know of any examples?

All locks? Something like:

	spin_lock(&x)
	/* foo */
	spin_unlock(&x)
	spin_lock(&x)
	/* bar */
	spin_unlock(&x);

Has a fairly high foo happens-before bar expectation level.

And in specific things like:

  135e8c9250dd5
  ecf7d01c229d1

which use the release of rq->lock paired with the next acquire of the
same rq->lock to match with an smp_rmb().