linux-kernel - Re: [PATCH v13 04/17] preempt: Introduce __preempt_count_{sub, add}

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87bjli7zg7.fsf@t14s.mail-host-address-is-not-set>
Date: Tue, 04 Nov 2025 13:30:00 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org, Thomas
 Gleixner <tglx@...utronix.de>, Boqun Feng <boqun.feng@...il.com>,
 linux-kernel@...r.kernel.org, Daniel Almeida
 <daniel.almeida@...labora.com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon
 <will@...nel.org>, Heiko Carstens <hca@...ux.ibm.com>, 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>, Jinjie Ruan
 <ruanjinjie@...wei.com>, Ada Couprie Diaz <ada.coupriediaz@....com>,
 Juergen Christ <jchrist@...ux.ibm.com>, Brian Gerst <brgerst@...il.com>,
 Uros Bizjak <ubizjak@...il.com>, "moderated list:ARM64 PORT (AARCH64
 ARCHITECTURE)" <linux-arm-kernel@...ts.infradead.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 v13 04/17] preempt: Introduce __preempt_count_{sub,
 add}_return()

Lyude Paul <lyude@...hat.com> writes:

> From: Boqun Feng <boqun.feng@...il.com>
>
> In order to use preempt_count() to tracking the interrupt disable
> nesting level, __preempt_count_{add,sub}_return() are introduced, as
> their name suggest, these primitives return the new value of the
> preempt_count() after changing it. The following example shows the usage
> of it in local_interrupt_disable():
>
> 	// increase the HARDIRQ_DISABLE bit
> 	new_count = __preempt_count_add_return(HARDIRQ_DISABLE_OFFSET);
>
> 	// if it's the first-time increment, then disable the interrupt
> 	// at hardware level.
> 	if (new_count & HARDIRQ_DISABLE_MASK == HARDIRQ_DISABLE_OFFSET) {
> 		local_irq_save(flags);
> 		raw_cpu_write(local_interrupt_disable_state.flags, flags);
> 	}
>
> Having these primitives will avoid a read of preempt_count() after
> changing preempt_count() on certain architectures.
>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
>
> ---
> V10:
> * Add commit message I forgot
> * Rebase against latest pcpu_hot changes
> V11:
> * Remove CONFIG_PROFILE_ALL_BRANCHES workaround from
>   __preempt_count_add_return()
>
>  arch/arm64/include/asm/preempt.h | 18 ++++++++++++++++++
>  arch/s390/include/asm/preempt.h  | 10 ++++++++++
>  arch/x86/include/asm/preempt.h   | 10 ++++++++++
>  include/asm-generic/preempt.h    | 14 ++++++++++++++
>  4 files changed, 52 insertions(+)
>
> diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
> index 932ea4b620428..0dd8221d1bef7 100644
> --- a/arch/arm64/include/asm/preempt.h
> +++ b/arch/arm64/include/asm/preempt.h
> @@ -55,6 +55,24 @@ static inline void __preempt_count_sub(int val)
>  	WRITE_ONCE(current_thread_info()->preempt.count, pc);
>  }
>  
> +static inline int __preempt_count_add_return(int val)
> +{
> +	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
> +	pc += val;
> +	WRITE_ONCE(current_thread_info()->preempt.count, pc);
> +
> +	return pc;
> +}
> +
> +static inline int __preempt_count_sub_return(int val)
> +{
> +	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
> +	pc -= val;
> +	WRITE_ONCE(current_thread_info()->preempt.count, pc);
> +
> +	return pc;
> +}
> +

I am wondering how this works when preemption is enabled? Will the
kernel never preempt itself? I would think this would have to be atomic?
I can see the surrounding code is using the same pattern, so it is
probably fine. But I am curious as to why that is.

Best regards,
Andreas Hindborg