Re: [PATCH] x86/split_lock: Zap the unwieldy switch-case in sld_state_show()

[Xiaoyao Li <xiaoyao.li@intel.com>]

+ Tony

On 1/5/2026 3:17 AM, Borislav Petkov wrote:
>  From e52fe2e2009e488c720f3e98a77963145ac9153c Mon Sep 17 00:00:00 2001
> From: "Borislav Petkov (AMD)"<bp@alien8.de>
> Date: Sun, 4 Jan 2026 14:40:23 +0100
> Subject: [PATCH] x86/split_lock: Zap the unwieldy switch-case in sld_state_show()
> 
> Handle the easy cases first and leave the meat of the code at the end,
> after having removed all possible gunk which makes it even more
> unreadable than it is.
> 
> Have the CPU-going-offline check for both fatal and warning settings
> because there's no point to have it only in the sld_warn case.

If I understand correctly, the CPU offline callback was added to avoid 
the case where a CPU is taken offline when split lock detection is 
disabled temporarily and before the delayed work to re-enable it being 
called. Since the MSR_TEST_CTRL is per-core scope, the sibling CPU on 
the same core may then be left running with split lock detection 
disabled without the delayed work to re-enable it.

For fatal mode, there is no such handling of temporarily disabling the 
feature and we don't need the CPU offline callback.

> There should be no functional changes resulting from this cleanup.
> 
> Signed-off-by: Borislav Petkov (AMD)<bp@alien8.de>
> ---
>   arch/x86/kernel/cpu/bus_lock.c | 39 ++++++++++++++++------------------
>   1 file changed, 18 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
> index fb166662bc0d..811f87906c1e 100644
> --- a/arch/x86/kernel/cpu/bus_lock.c
> +++ b/arch/x86/kernel/cpu/bus_lock.c
> @@ -391,34 +391,31 @@ static void __init split_lock_setup(struct cpuinfo_x86 *c)
>   
>   static void sld_state_show(void)
>   {
> +	const char *action = "warning";
> +
>   	if (!boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT) &&
>   	    !boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
>   		return;
>   
> -	switch (sld_state) {
> -	case sld_off:
> +	if (sld_state == sld_off) {
>   		pr_info("disabled\n");
> -		break;
> -	case sld_warn:
> -		if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
> -			pr_info("#AC: crashing the kernel on kernel split_locks and warning on user-space split_locks\n");
> -			if (cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> -					      "x86/splitlock", NULL, splitlock_cpu_offline) < 0)
> -				pr_warn("No splitlock CPU offline handler\n");
> -		} else if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT)) {
> -			pr_info("#DB: warning on user-space bus_locks\n");
> -		}
> -		break;
> -	case sld_fatal:
> -		if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
> -			pr_info("#AC: crashing the kernel on kernel split_locks and sending SIGBUS on user-space split_locks\n");
> -		else if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
> -			pr_info("#DB: sending SIGBUS on user-space bus_locks\n");
> -		break;
> -	case sld_ratelimit:
> +		return;
> +	} else if (sld_state == sld_ratelimit) {
>   		if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
>   			pr_info("#DB: setting system wide bus lock rate limit to %u/sec\n", bld_ratelimit.burst);
> -		break;
> +		return;
> +	}
> +
> +	if (sld_state == sld_fatal)
> +		action = "sending SIGBUS";
> +
> +	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
> +		pr_info("#AC: crashing the kernel on kernel split_locks and %s on user-space split_locks\n", action);
> +		if (cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> +				      "x86/splitlock", NULL, splitlock_cpu_offline) < 0)
> +			pr_warn("No splitlock CPU offline handler\n");
> +	} else if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT)) {
> +		pr_info("#DB: %s on user-space bus_locks\n", action);
>   	}
>   }
>   
> -- 2.51.0