Re: [PATCH 3/3 v5] s390/pai: Support CPU hotplug for PMU PAI

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Calling `cpus_read_lock()` inside CPU hotplug callbacks causes a guaranteed self-deadlock.
--

--- Patch [3]: [PATCH 3/3 v5] s390/pai: Support CPU hotplug for PMU PAI ---
commit c7c540e2c7341f700711120ef64b2203ca9970ed
Author: Thomas Richter <tmricht@linux.ibm.com>
s390/pai: Support CPU hotplug for PMU PAI

This patch adds CPU hotplug support for the PAI PMU driver. It allocates and
frees per-CPU data structures during CPU online and offline operations to
prevent a kernel panic when PMU callbacks run on newly onlined CPUs.

> diff --git a/arch/s390/kernel/perf_pai.c b/arch/s390/kernel/perf_pai.c
> --- a/arch/s390/kernel/perf_pai.c
> +++ b/arch/s390/kernel/perf_pai.c

[ ... ]

> @@ -1216,8 +1256,38 @@ static int __init paipmu_setup(void)
>  	return install_ok;
>  }
>  
> +static int pai_online_cpu(unsigned int cpu)
> +{
> +	int rc;
> +
> +	cpus_read_lock();

[Severity: High]
Will this cause a self-deadlock? The CPU hotplug core already holds the
cpu_hotplug_lock for writing when it invokes the hotplug callbacks.
Attempting to acquire it for reading via cpus_read_lock() here seems like
it would cause a deadlock, as percpu_rwsem does not support recursive
read-after-write locking.

> +	mutex_lock(&pai_reserve_mutex);
> +	rc = pai_alloc_cpu(PAI_PMU_CRYPTO, cpu, true);
> +	if (!rc) {
> +		rc = pai_alloc_cpu(PAI_PMU_EXT, cpu, true);
> +		if (rc)
> +			pai_event_destroy_cpu(PAI_PMU_CRYPTO, cpu, true);
> +	}
> +	mutex_unlock(&pai_reserve_mutex);
> +	cpus_read_unlock();
> +	return rc;
> +}
> +
> +static int pai_offline_cpu(unsigned int cpu)
> +{
> +	cpus_read_lock();

[Severity: High]
Similarly, does calling cpus_read_lock() here in the offline callback cause
a self-deadlock for the same reason?

> +	mutex_lock(&pai_reserve_mutex);
> +	pai_event_destroy_cpu(PAI_PMU_CRYPTO, cpu, true);
> +	pai_event_destroy_cpu(PAI_PMU_EXT, cpu, true);
> +	mutex_unlock(&pai_reserve_mutex);
> +	cpus_read_unlock();
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260824131751.179275-1-tmricht@linux.ibm.com?part=3