[PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

Christian Borntraeger
Fri, 25 Nov 2016 16:49:45 -0000

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().
>