----- On Jul 3, 2018, at 1:34 PM, Peter Zijlstra pet...@infradead.org wrote:
> On Tue, Jul 03, 2018 at 10:10:37AM -0700, Linus Torvalds wrote: >> On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <a...@firstfloor.org> wrote: >> > >> > So it sounds like architectures that don't have an instruction atomic u64 >> > *_user need to disable interrupts during the access, and somehow handle >> > that >> > case when a page fault happens? >> >> No. It's actually the store by *user* space that is the critical one. >> Not the whole 64-bit value, just the low pointer part. >> >> The kernel could do it as a byte-by-byte load, really. It's >> per-thread, and once the kernel is running, it's not going to change. >> The kernel never changes the value, it just loads it from user space. > > The kernel doesn't change _this_ value, but the kernel does change other > values, like for instance rseq->cpu_id. But even there, it could use > byte stores and it is again the userspace load of that field that is > critical again and needs to be a single op. I can simply document that loads/stores from/to all struct rseq fields should be thread-local then ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
