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

[Christian Borntraeger]

On 11/25/2016 06:28 PM, Mark Rutland wrote:
> On Fri, Nov 25, 2016 at 05:49:45PM +0100, Christian Borntraeger wrote:
>> 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).
> 
> We have similar on 32-bit ARM w/ LPAE. LPAE implies that a naturally
> aligned 64-bit access is single-copy atomic, which is what makes that
> ok.
> 
>> And the mm/ code is perfectly fine with these PTE accesses being done
>> NOT atomic.
> 
> That strikes me as surprising. Is there some mutual exclusion that
> prevents writes from occuring wherever a READ_ONCE() happens to a PTE?

See for example mm/memory.c handle_pte_fault.

---snip----

                /*
                 * some architectures can have larger ptes than wordsize,
                 * e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and
                 * CONFIG_32BIT=y, so READ_ONCE or ACCESS_ONCE cannot guarantee
                 * atomic accesses.  The code below just needs a consistent
                 * view for the ifs and we later double check anyway with the
                 * ptl lock held. So here a barrier will do.
                 */
---snip---

The trick is that the code only does a specific check, but all other accesses are under
the pte lock.