----- On Jul 3, 2018, at 1:10 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx 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. > > So all the atomicity worries for the kernel are a red herring. They'd > arguably be nice to have - but only for an insane case that makes > absolutely no sense (a different thread trying to change the value). > > Can we please stop the idiocy already? The kernel could read the rseq > pointer one bit at a time, and do a little dance with "yield()" in > between, and take interrupts and page faults, and it wouldn't matter > AT ALL. > > It's not even that we read the value from an interrupt context, it's > that as we return to user space (which can be the result of an > interrupt) we can read the value. > > This whole thread has been filled with crazy "what if" things that don't matter. Sorry to come back in the thread late, looks like I've missed all the fun. I agree with Linus: we can simply document that updates to rseq->rseq_cs should be thread-local in the rseq uapi and be done with it. This would allow using get_user(u64) even on 32-bit architectures, because we cannot care less if an architecture chooses to read the u64 byte-wise while standing on its feet. With this added requirement, Andy's idea of using a union between __u64 and upper/lower __u32 would fit very nicely. If everyone is OK with that approach, I can prepare an updated patch. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- 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
