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

On Fri, Nov 25, 2016 at 05:28:01PM +0000, Mark Rutland wrote:

On Fri, Nov 25, 2016 at 05:49:45PM +0100, Christian Borntraeger wrote:

quoted

On 11/25/2016 05:17 PM, Peter Zijlstra wrote:

quoted

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.

quoted

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?

Otherwise, how is tearing not a problem? Does it have some pattern like
the lockref cmpxchg?

On x86 PAE we play silly games, see arch/x86/mm/gup.c:gup_get_ptr().

Those two loads really should be READ_ONCE()/LOAD_SINGLE().