Re: [PATCH 4/7] virt: steal_governor: serialize sampling against CPU hotplug

[sashiko-bot@kernel.org]

> Steal governor is not safe against a concurrent CPU hotplug operation
> because it can change active and preferred cpumasks during the governor
> sampling.
> 
> To address the race, the governor would take the cpus_write_lock(). But
> that may be impractical for the loadable module.
> 
> Instead, implement the following locking policy:
> 
>  - if the code takes cpus_write_lock(), it's enough to write-access
>    the preferred cpumask;
>  - if the code takes cpus_read_lock(), it must also take
>    cpu_preferred_mutex in order to modify the preferred mask.
> 
> The core scheduler and hotplug paths already modify the preferred cpumask
> under the cpus_write_lock(), so no change in core logic is required.
> 
> This provides a common update protocol for multiple policy drivers
> without requiring writers to take the broader CPU hotplug write lock.
> 
> Fixes: 27d47ebce4d6 ("virt/steal_governor: Implement steal_governor policy loop")
> Signed-off-by: Yury Norov <ynorov@nvidia.com>

Sashiko has reviewed this patch and found no issues.

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