linux-kernel - Re: [PATCH v9 2/9] preempt: Introduce __preempt_count_{sub, add}

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Message-ID: <20250228092405.GE5880@noisy.programming.kicks-ass.net>
Date: Fri, 28 Feb 2025 10:24:05 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Boqun Feng <boqun.feng@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Juergen Christ <jchrist@...ux.ibm.com>,
	Ilya Leoshkevich <iii@...ux.ibm.com>,
	"moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" <linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:S390 ARCHITECTURE" <linux-s390@...r.kernel.org>,
	"open list:GENERIC INCLUDE/ASM HEADER FILES" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v9 2/9] preempt: Introduce __preempt_count_{sub,
 add}_return()

On Fri, Feb 28, 2025 at 10:15:09AM +0100, Heiko Carstens wrote:

> I couldn't find any cover letter for the whole patch series which describes
> what this is about, and why it is needed.
> It looks like some Rust enablement?

Yeah, more or less.

It's replacing local_irq_save() and all related functions
(spin_lock_irqsave etc..) that take a flags argument with this new thing
that frobs a recursion count in preempt_count(), obviating the need to
carry the local flags argument around.

This is nice, even for C code, less flags muck to carry around.

It would be even better if they then went and deleted all of the _irq /
_irqsave nonsense entirely.

Yes, that's going to be a big patch :-)

Also, IIRC there is some arch stuff that comes unstuck if you do this
blindly (I tried at some point, it didn't boot).