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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Wed, 22 Nov 2023 21:12:04 +0100
Message-ID: <CAJZ5v0jbjykNooJ_PvkOjpE6hA0QFJUw+kpwYfNY7jOtkcPA0Q@mail.gmail.com>
Subject: Re: [PATCH 02/21] x86: intel_epb: Don't rely on link order
To:     Russell King <rmk+kernel@armlinux.org.uk>
Cc:     linux-pm@vger.kernel.org, loongarch@lists.linux.dev,
        linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org,
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        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        jianyong.wu@arm.com, justin.he@arm.com,
        James Morse <james.morse@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Tue, Nov 21, 2023 at 2:44 PM Russell King <rmk+kernel@armlinux.org.uk> wrote:
>
> From: James Morse <james.morse@arm.com>
>
> intel_epb_init() is called as a subsys_initcall() to register cpuhp
> callbacks. The callbacks make use of get_cpu_device() which will return
> NULL unless register_cpu() has been called. register_cpu() is called
> from topology_init(), which is also a subsys_initcall().
>
> This is fragile. Moving the register_cpu() to a different
> subsys_initcall()  leads to a NULL dereference during boot.
>
> Make intel_epb_init() a late_initcall(), user-space can't provide a
> policy before this point anyway.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

and I'd suggest sending this separately to the x86 list.

> ---
> subsys_initcall_sync() would be an option, but moving the register_cpu()
> calls into ACPI also means adding a safety net for CPUs that are online
> but not described properly by firmware. This lives in subsys_initcall_sync().
> ---
>  arch/x86/kernel/cpu/intel_epb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
> index e4c3ba91321c..f18d35fe27a9 100644
> --- a/arch/x86/kernel/cpu/intel_epb.c
> +++ b/arch/x86/kernel/cpu/intel_epb.c
> @@ -237,4 +237,4 @@ static __init int intel_epb_init(void)
>         cpuhp_remove_state(CPUHP_AP_X86_INTEL_EPB_ONLINE);
>         return ret;
>  }
> -subsys_initcall(intel_epb_init);
> +late_initcall(intel_epb_init);
> --
> 2.30.2
>
>