Re: [PATCH v2 00/12] Refcounted interrupt disable and SpinLockIrq for rust (Part 1)

[Peter Zijlstra]

On Tue, May 26, 2026 at 08:21:36AM -0700, Boqun Feng wrote:
> Hi Peter,
> 
> This is a follow-up for Lyude's work [1]. After learning the current
> preempt_count() usage and how ARM64 handle this, I came up with this
> series that could resolve your feedback [2]. The basic idea is based on:
> 
> 1) preempt_count() previously already masks our NEED_RESCHED bit, so the
>    effective bits is 31bits
> 2) with a 64bit preempt count implementation (as in your PREEMPT_LONG
>    proposal), the effective bits that record "whether we CAN preempt or
>    not" still fit in 32bit (i.e. an int)
> 
> as a result, I don't think we need to change the existing
> preempt_count() API, but rather keep "32bit vs 64bit" as an
> implementation detail. This saves us the need to change the printk code
> for preempt_count().

> 
> v1: https://lore.kernel.org/rust-for-linux/20260508042111.24358-1-boqun@xxxxxxxxxx/
> 
> Changes since v1:
> 
> * Rename PREEMPT_COUNT_64BIT to HAS_SEPARATE_PREEMPT_RESCHED_BITS per
>   Mark Rutland.

Blergh, so I really don't like that new name. It isn't that
PREEMPT_RESCHED is separate, it really is a 64bit preempt count.

Shashiko has a few fits, but its mostly being stupid. Although I think
it might be useful to perhaps put a WARN_ON_ONCE(in_nmi()) in
local_interrupt_disable().

Anyway, I'll re-read things again tomorrow, but I suppose this will do.