On Tue, Jul 03, 2018 at 10:10:37AM -0700, Linus Torvalds wrote: > On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <andi@xxxxxxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
