Re: [PATCH 2/3] sched_ext: Track bits[] storage size in struct scx_cmask

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The per-CPU `scx_set_cmask_scratch` variable is never initialized, causing `scx_cpumask_to_cmask()` to silently pass empty CPU masks to the BPF scheduler.
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commit c63d115e3f87dfb00acdde042926edb85a5907eb
Author: Tejun Heo <tj@kernel.org>

sched_ext: Track bits[] storage size in struct scx_cmask

This commit adds an alloc_words field to struct scx_cmask to track the
allocation size of the bits[] array, and splits the bit-range API into 
initialization, reframing, and clearing helpers.

> diff --git a/kernel/sched/ext_cid.c b/kernel/sched/ext_cid.c
> index bdd8ef8eae3dc..f50319c5c65e1 100644
> --- a/kernel/sched/ext_cid.c
> +++ b/kernel/sched/ext_cid.c

[ ... ]

> @@ -275,10 +275,10 @@ void scx_cpumask_to_cmask(const struct cpumask *src, struct scx_cmask *dst)
>  {
>  	s32 cpu;
>  
> -	scx_cmask_init(dst, 0, num_possible_cpus());
> +	scx_cmask_clear(dst);

Can this regression cause scx_cpumask_to_cmask() to generate empty masks
when dst is scx_set_cmask_scratch?

Since scx_set_cmask_scratch is allocated zero-filled, its nr_cids defaults
to 0.

With scx_cmask_init() removed here, if scx_set_cmask_scratch is passed as
dst without being initialized elsewhere, scx_cmask_clear() will see
!dst->nr_cids and return immediately doing nothing.

The subsequent loop then calls __scx_cmask_set(dst, cid) for each CPU.
Because __scx_cmask_contains() evaluates to cid >= 0 && cid < 0, which is
always false, no bits are ever set.

Could this result in the BPF scheduler receiving an empty cmask for
operations like set_cpumask?

>  	for_each_cpu(cpu, src) {
>  		s32 cid = __scx_cpu_to_cid(cpu);

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Sashiko AI review · https://sashiko.dev/#/patchset/20260517181022.1184056-1-tj@kernel.org?part=2