linux-kernel - Re: [PATCH] cpu: fix hard lockup triggered by printk calls within scheduling context

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Message-ID: <aN_ccUJZQVMmk1V-@willie-the-truck>
Date: Fri, 3 Oct 2025 15:23:45 +0100
From: Will Deacon <will@...nel.org>
To: shechenglong <shechenglong@...sion.com>
Cc: catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, stone.xulei@...sion.com,
	chenjialong@...sion.com, yuxiating@...sion.com,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] cpu: fix hard lockup triggered by printk calls within
 scheduling context

On Wed, Sep 24, 2025 at 08:32:47PM +0800, shechenglong wrote:
> relocate the printk() calls from spectre_v4_mitigations_off() and
> spectre_v2_mitigations_off() into setup_system_capabilities() function,
> preventing hard lockups that occur when printk() is invoked from scheduler context.
> 
> Link: https://patchwork.kernel.org/project/linux-arm-kernel/patch/20250918064907.1832-1-shechenglong@xfusion.com/
> Suggested-by: Mark Rutland <mark.rutland@....com>
> Suggested-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: shechenglong <shechenglong@...sion.com>
> ---
>  arch/arm64/include/asm/spectre.h |  3 +++
>  arch/arm64/kernel/cpufeature.c   |  9 +++++++++
>  arch/arm64/kernel/proton-pack.c  | 18 ++++--------------
>  3 files changed, 16 insertions(+), 14 deletions(-)

Thanks for posting the patch!

> diff --git a/arch/arm64/include/asm/spectre.h b/arch/arm64/include/asm/spectre.h
> index 8fef12626090..6fe29df41788 100644
> --- a/arch/arm64/include/asm/spectre.h
> +++ b/arch/arm64/include/asm/spectre.h
> @@ -118,5 +118,8 @@ void spectre_bhb_patch_wa3(struct alt_instr *alt,
>  void spectre_bhb_patch_clearbhb(struct alt_instr *alt,
>  				__le32 *origptr, __le32 *updptr, int nr_inst);
>  
> +bool spectre_v2_mitigations_off(void);
> +bool spectre_v4_mitigations_off(void);
> +
>  #endif	/* __ASSEMBLY__ */
>  #endif	/* __ASM_SPECTRE_H */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index ef269a5a37e1..7d1f541e66a0 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -94,6 +94,7 @@
>  #include <asm/traps.h>
>  #include <asm/vectors.h>
>  #include <asm/virt.h>
> +#include <asm/spectre.h>
>  
>  /* Kernel representation of AT_HWCAP and AT_HWCAP2 */
>  static DECLARE_BITMAP(elf_hwcap, MAX_CPU_FEATURES) __read_mostly;
> @@ -3942,6 +3943,14 @@ static void __init setup_system_capabilities(void)
>  	 */
>  	if (system_uses_ttbr0_pan())
>  		pr_info("emulated: Privileged Access Never (PAN) using TTBR0_EL1 switching\n");
> +
> +	/*
> +	 * Report Spectre mitigations status.
> +	 */
> +	if (spectre_v2_mitigations_off())
> +		pr_info("spectre-v2 mitigation disabled by command line option\n");

nit: Let's fix the message here for consistency and use "command-line".

> +	if (spectre_v4_mitigations_off())
> +		pr_info("spectre-v4 mitigation disabled by command-line option\n");

I think it would be cleaner to have a new function in proton-pack.c, say
"spectre_print_disabled_mitigations()" which can then print the messages
for spectre v2, v4 and also spectre-bhb (currently done in
spectre_bhb_enable_mitigation()).

While you're at it, spectre-bhb is weird because it also prints a message
when the Kconfig is disabled. We should actually just get rid of that
Kconfig option altogether (in a separate patch) like we did for the other
spectre mitigations.

What do you think?

Cheers,

Will