Re: [PATCH bpf-next v2 2/3] selftests/bpf: libarena: Add spmc queue data structure

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Duplicate pops occur in spmc_owned_remove.
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commit 32e23022b6cf8e4bb5da76036f824fc171bf9e71
Author: Emil Tsalapatis <emil@etsalapatis.com>

selftests/bpf: libarena: Add spmc queue data structure

Expand libarena with a single producer multiple consumer deque data
structure. This is a single producer, multiple consumer lockless structure
that permits efficient work stealing. The structure is a Lev-Chase queue,
so it is lock-free and wait-free.

> diff --git a/tools/testing/selftests/bpf/libarena/src/spmc.bpf.c b/tools/testing/selftests/bpf/libarena/src/spmc.bpf.c
> new file mode 100644
> index 0000000000000..7fd7b18a03732
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/libarena/src/spmc.bpf.c
[ ... ]
> +__weak
> +int spmc_owned_remove(struct spmc __arena *spmc, u64 *val)
> +{
> +	volatile struct spmc_arr __arena *arr;
> +	int ret = 0;
> +	ssize_t sz;
> +	u64 value;
> +	u64 b, t;
> +
> +	if (unlikely(!spmc || !val))
> +		return -EINVAL;
> +
> +	b = smp_load_acquire(&spmc->bottom) - 1;
> +	t = smp_load_acquire(&spmc->top);
> +	arr = READ_ONCE(spmc->cur);
> +
> +	smp_store_release(&spmc->bottom, b);

[Severity: Medium]
Could this ordering result in duplicate pops?

Because top is read before bottom is updated here, could stealers erroneously
consume the same item as the owner? In a standard Chase-Lev queue, storing
bottom before evaluating top, separated by a full memory barrier, is required
to avoid this race.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260603182727.3922-1-emil@etsalapatis.com?part=2