> On Sep 26, 2018, at 10:03 AM, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote: > >> On Wed, Sep 26, 2018 at 6:21 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> Are, is what you’re saying that the Zinc chacha20 functions should call simd_relax() every n bytes automatically for some reasonable value of n? If so, seems sensible, except that some care might be needed to make sure they interact with preemption correctly. >> >> What I mean is: the public Zinc entry points should either be callable in an atomic context or they should not be. I think this should be checked at runtime in an appropriate place with an __might_sleep or similar. Or simd_relax should learn to *not* schedule if the result of preempt_enable() leaves it atomic. (And the latter needs to be done in a way that works even on non-preempt kernels, and I don’t remember whether that’s possible.). And this should happen regardless of how many bytes are processed. IOW, calling into Zinc should be equally not atomic-safe for 100 bytes and for 10 MB. > > I'm not sure this is actually a problem. Namely: > > preempt_disable(); > kernel_fpu_begin(); > kernel_fpu_end(); > schedule(); <--- bug! > > Calling kernel_fpu_end() disables preemption, but AFAIK, preemption > enabling/disabling is recursive, so kernel_fpu_end's use of > preempt_disable won't actually do anything until the outer preempt > enable is called: > > preempt_disable(); > kernel_fpu_begin(); > kernel_fpu_end(); > preempt_enable(); > schedule(); <--- works! > > Or am I missing some more subtle point? > No, I think you’re right. I was mid-remembering precisely how simd_relax() worked.
