Re: [PATCH 7/7] cpu: make preferred CPU policy writers mutually exclusive

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] cpumask: preferred_cpumask_owned_by(NULL) returns true when
  unclaimed
--

commit 62477968d02f2aaa9afd9c7a8010ab357c98eb83
Author: Yury Norov <ynorov@nvidia.com>

cpu: make preferred CPU policy writers mutually exclusive

This commit adds helpers to claim, release, and check exclusive ownership of
the preferred CPU mask, and updates steal_governor to use them.

> diff --git a/lib/cpumask.c b/lib/cpumask.c
> index 2e16e19a2bdb6..ee710c3c89201 100644
> --- a/lib/cpumask.c
> +++ b/lib/cpumask.c
[ ... ]
> @@ -170,4 +172,64 @@ EXPORT_SYMBOL(cpumask_any_distribute);
>  #ifdef CONFIG_PREFERRED_CPU
>  DEFINE_MUTEX(cpu_preferred_mutex);
>  EXPORT_SYMBOL_GPL(cpu_preferred_mutex);
> +
> +static const void *preferred_cpumask_owner;
> +
> +/**
> + * preferred_cpumask_owned_by - test preferred CPU policy ownership
> + * @owner: policy identity
> + *
> + * Return: true if @owner holds the policy, false otherwise.
> + */
> +bool preferred_cpumask_owned_by(const void *owner)
> +{
> +	lockdep_assert_held(&cpu_preferred_mutex);
> +
> +	return preferred_cpumask_owner == owner;
> +}
> +EXPORT_SYMBOL_GPL(preferred_cpumask_owned_by);
> +
> +/**
> + * claim_preferred_cpumask - acquire exclusive preferred CPU policy ownership
> + * @owner: non-NULL identity that remains valid until ownership is released
> + *
> + * Call before starting policy work, normally on module init.
> + *
> + * Return: 0 on success, -EBUSY if claimed, or -EINVAL for a NULL identity.
> + */
> +int claim_preferred_cpumask(const void *owner)
> +{
> +	if (!owner)
> +		return -EINVAL;
> +
> +	guard(cpus_read_lock)();
> +	guard(mutex)(&cpu_preferred_mutex);
> +
> +	if (preferred_cpumask_owner)
> +		return -EBUSY;
> +
> +	preferred_cpumask_owner = owner;
> +	reset_preferred_cpumask();
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(claim_preferred_cpumask);
> +
> +/**
> + * release_preferred_cpumask - release exclusive preferred CPU policy ownership
> + * @owner: identity used to claim ownership
> + *
> + * Call after stopping policy work, normally on module exit.
> + */
> +void release_preferred_cpumask(const void *owner)
> +{
> +	guard(cpus_read_lock)();
> +	guard(mutex)(&cpu_preferred_mutex);
> +
> +	if (WARN_ON_ONCE(!owner || !preferred_cpumask_owned_by(owner)))
> +		return;

[Severity: Low]
Does preferred_cpumask_owned_by(NULL) return true when no policy driver owns
the preferred cpumask?  Checking owner && preferred_cpumask_owner == owner in
preferred_cpumask_owned_by() would also avoid needing the separate !owner
check in release_preferred_cpumask().

Also, the kernel-doc comments for preferred_cpumask_owned_by(),
claim_preferred_cpumask(), and release_preferred_cpumask() do not mention
their locking expectations (holding cpu_preferred_mutex for
preferred_cpumask_owned_by(), and acquiring cpus_read_lock() and
cpu_preferred_mutex internally in the claim/release helpers).

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113184257.621707-2-ynorov@nvidia.com?part=7