Re: [PATCH 2/2] arm64: preempt: Provide our own implementation of asm/preempt.h

[Peter Zijlstra]

On Wed, Nov 28, 2018 at 04:35:42PM +0100, Ard Biesheuvel wrote:
> On Tue, 27 Nov 2018 at 20:44, Will Deacon <will.deacon@xxxxxxx> wrote:
> >
> > The asm-generic/preempt.h implementation doesn't make use of the
> > PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store
> > architectures which rely on the preempt_count word being unchanged across
> > an interrupt.
> >
> > However, since we're a 64-bit architecture and the preempt count is
> > only 32 bits wide, we can simply pack it next to the resched flag and
> > load the whole thing in one go, so that a dec-and-test operation doesn't
> > need to load twice.
> >
> 
> Since the actual preempt count is a lot narrower than 32 bits, x86
> just uses bit 31.
> 
> So what is the reason for using two different words?

See commit:

  ba1f14fbe709 ("sched: Remove PREEMPT_NEED_RESCHED from generic code")