----- On Jul 3, 2018, at 1:48 PM, Peter Zijlstra pet...@infradead.org wrote:
> On Tue, Jul 03, 2018 at 01:38:59PM -0400, Mathieu Desnoyers wrote: >> ----- 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 ? > > I'm not sure that covers things sufficiently. You really want the > userspace load/stores to be single instructions. Yes, of course. More specifically, I would document that those need to be single-copy atomicity load/store performed by the local thread. > Also, I think it was rseq_update_cpu_id() where we wanted to use a > single u64 store if possible but you worried about the stores. With this added bit of restriction on thread-local loads, indeed we can then update them without caring about atomicity at kernel level. I can modify the ABI to put the cpu_id_start and cpu_id fields inside a union, and update it with a single store. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
