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? Otherwise, how is tearing not a problem? Does it have some pattern like the lockref cmpxchg? Thanks, Mark. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
