Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations

[Jason A. Donenfeld]

On Wed, Sep 26, 2018 at 6:21 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> Are, is what youâre 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 donât remember whether thatâs 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?

Jason