Re: [PATCH 2/2] selftests/sched_ext: Check the cmask cid-form ops.enable() receives

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] selftests/sched_ext: data race on global error variables in check_mask()
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commit 72058557dd9acff8213206e33ae6e84591889e86
Author: Tejun Heo <tj@kernel.org>

selftests/sched_ext: Check the cmask cid-form ops.enable() receives

This commit adds a new cid-form selftest to verify that ops.enable()
receives the correct task cmask. It checks the mask against p->cpus_ptr
and ensures that the initial set_cmask() matches enable() and is called
in the correct sequence.

> diff --git a/tools/testing/selftests/sched_ext/enable_cmask.bpf.c b/tools/testing/selftests/sched_ext/enable_cmask.bpf.c
> new file mode 100644
> index 0000000000000..bbbf989d2c698
> --- /dev/null
> +++ b/tools/testing/selftests/sched_ext/enable_cmask.bpf.c

[ ... ]

> +u64 nr_enable, nr_initial_set_cmask, nr_set_cmask, nr_set_weight;
> +s32 bad_cid;
> +bool bad_want, bad_got;
> +
> +UEI_DEFINE(uei);

[ ... ]

> +static int check_mask(struct task_struct *p, const struct scx_cmask __arena *m, u64 *fp)
> +{
> +	u32 nr_cids = scx_bpf_nr_cids();
> +	u64 h = 0;
> +	s32 cid;
> +
> +	if (m->base || m->nr_cids != nr_cids)
> +		return -EINVAL;
> +
> +	bpf_for(cid, 0, MAX_CPUS) {
> +		bool want, got;
> +		s32 cpu;
> +
> +		if (cid >= nr_cids)
> +			break;
> +		cpu = scx_bpf_cid_to_cpu(cid);
> +		if (cpu < 0)
> +			return -ENOENT;
> +		want = bpf_cpumask_test_cpu(cpu, p->cpus_ptr);
> +		got = cmask_test(cid, m);
> +		if (want != got) {
> +			bad_cid = cid;
> +			bad_want = want;
> +			bad_got = got;
> +			return -EIO;
> +		}

[Severity: Low]
Can these global error variables be overwritten concurrently if multiple tasks
fail the cpumask check at the same time?

Since bad_cid, bad_want, and bad_got are global and updated without atomics 
or locks, a race condition here might lead to mismatched or incorrect values 
being logged in the error message when the test is failing.

> +		h = h * 31 + got;
> +	}
> +
> +	*fp = h;
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260919002838.1960071-1-tj@kernel.org?part=2