[PATCH v2 3/5] ARM: add support for kernel mode NEON

[Ard Biesheuvel]

On 26 June 2013 14:40, Will Deacon <will.deacon at arm.com> wrote:
> On Wed, Jun 26, 2013 at 12:28:49PM +0100, Ard Biesheuvel wrote:
>> So can you suggest a better way of making sure schedule_debug() shoots
>> us down if calling schedule() between kernel_neon_begin and
>> kernel_neon_end, even on non-preempt builds?
>
> With what we currently have in the kernel, no, I can't think of a better
> way. However, I also don't think that smuggling in a back-end hack is a good
> idea either. How about we follow x86's lead on this and rely on the caller
> not to sleep for the timebeing? Then, separately to this patch series, you
> could look at augmenting the scheduler so that schedule_debug can complain
> if it encounters a task that is not expected to sleep? That seems like the
> right place to fix this problem, and will benefit other architectures too.
>

Good point. As preempt_enable/disable already have this side effect on
PREEMPT builds, perhaps it wouldn't be such a bad idea to modify them
in the non-PREEMPT case to at least complain if schedule() is invoked
during such a section.

@Russell: you mentioned spinlocks at some point to prevent sleeping.
Are you ok with Will's suggestion instead, i.e., to rely on
preempt_disable() to do the right thing, and fix it later because
currently, it doesn't on non-PREEMPT?

-- 
Ard.