From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
Date: Fri, 25 Nov 2016 17:49:45 +0100
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Cc: Davidlohr Bueso <dave@stgolabs.net>, KVM list <kvm@vger.kernel.org>,
	dbueso@suse.de, netdev <netdev@vger.kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>, "Michael S. Tsirkin" <mst@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	virtualization@lists.linux-foundation.org,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Dmitry Vyukov <dvyukov@google.com>
To: Peter Zijlstra <peterz@infradead.org>, Mark Rutland <mark.rutland@arm.com>
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On 11/25/2016 05:17 PM, Peter Zijlstra wrote:
> On Fri, Nov 25, 2016 at 04:10:04PM +0000, Mark Rutland wrote:
>> On Fri, Nov 25, 2016 at 04:21:39PM +0100, Dmitry Vyukov wrote:
> 
>>> What are use cases for such primitive that won't be OK with "read once
>>> _and_ atomically"?
>>
>> I have none to hand.
> 
> Whatever triggers the __builtin_memcpy() paths, and even the size==8
> paths on 32bit.
> 
> You could put a WARN in there to easily find them.

There were several cases that I found during writing the *ONCE stuff.
For example there are some 32bit ppc variants with 64bit PTEs. Some for
others (I think sparc). And the mm/ code is perfectly fine with these
PTE accesses being done NOT atomic.


> 
> The advantage of introducing the SINGLE_{LOAD,STORE}() helpers is that
> they compiletime validate this the size is 'right' and can runtime check
> alignment constraints.
> 
> IE, they are strictly stronger than {READ,WRITE}_ONCE().
>