Re: [PATCH] s390/smp: Reflect (de)configured CPUs to cpu_enabled_mask

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Logical CPU ID fragmentation causes smp_get_base_cpu() to return incorrect siblings, leading to unsafe core deconfiguration.
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--- Patch [1]: [PATCH] s390/smp: Reflect (de)configured CPUs to cpu_enabled_mask ---
commit d82385b96c4cbe6b55b4a0446d3eeb4e2c122fa2
Author: Mete Durlu <meted@linux.ibm.com>

s390/smp: Reflect (de)configured CPUs to cpu_enabled_mask

On s390, CPUs can be in a state where it is not possible to hotplug
them online before certain prequisite steps. This commit adds necessary
checks to smp_add_core() and userspace configure attribute handlers to
reflect the configured CPUs to cpu_enabled_mask.

> diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
> index 0ba7f89b81611..e0c9480c7bd24 100644
> --- a/arch/s390/kernel/smp.c
> +++ b/arch/s390/kernel/smp.c
> @@ -1039,6 +1039,7 @@ static ssize_t cpu_configure_store(struct device *dev,
>  			per_cpu(pcpu_devices, cpu + i).state = CPU_STATE_STANDBY;
>  			smp_cpu_set_polarization(cpu + i,
>  						 POLARIZATION_UNKNOWN);
> +			set_cpu_enabled(cpu + i, false);

[Severity: High]
This isn't a bug introduced by this patch, but does the base CPU calculation
safely handle logical CPU ID fragmentation?

If arch_register_cpu() previously failed (e.g., due to memory allocation)
for a thread during hotplug in smp_add_core(), cpu_present is cleared. Upon
the next CPU rescan, __smp_rescan_cpus() repopulates the avail mask with
this unaligned hole.

When a new core is added, it will consume this hole and subsequent bits for
its threads, resulting in unaligned logical CPU IDs. However,
smp_get_base_cpu() mathematically assumes rigid alignment:

arch/s390/include/asm/smp.h:smp_get_base_cpu() {
	return cpu - (cpu % (smp_cpu_mtid + 1));
}

If a fragmented core is configured or deconfigured via sysfs, this
misalignment causes cpu_configure_store() to compute the wrong base CPU.

Could this lead to checking cpu_online() on unrelated CPUs, deconfiguring
the wrong hardware address via sclp_core_deconfigure(), and corrupting the
state and cpu_enabled_mask for unrelated CPUs?

>  		}
>  		topology_expect_change();
>  		break;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260723-enabled_cpus-v1-1-3a2cbdc374b0@linux.ibm.com?part=1