linux-kernel - Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic

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Date:   Wed, 15 Nov 2017 14:15:19 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Will Deacon <will.deacon@....com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        <parri.andrea@...il.com>, <boqun.feng@...il.com>,
        <dhowells@...hat.com>, <david@...morbit.com>
Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in
 atomic_t

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

Alan