Re: [PATCH RESEND bpf-next v3 6/6] selftests/bpf: libarena: Add parallel bitmap selftest

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Spurious test failure due to BPF instruction budget exhaustion during atomic operation retries
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commit 03c6a4c41937c3d9d257316bef5f1cacfa40aa71
Author: Emil Tsalapatis <emil@etsalapatis.com>

selftests/bpf: libarena: Add parallel bitmap selftest
    
This commit introduces a new selftest for the libarena atomic bitmap
operations. It spawns two threads that concurrently modify even and odd
bits to verify that operations do not improperly affect adjacent bits.

> diff --git a/tools/testing/selftests/bpf/libarena/selftests/test_parallel_bitmap.bpf.c b/tools/testing/selftests/bpf/libarena/selftests/test_parallel_bitmap.bpf.c
> new file mode 100644
> index 0000000000000..eec2871e2b0d1
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/libarena/selftests/test_parallel_bitmap.bpf.c

[ ... ]

> +__weak
> +int bitmap_test_bit_sequence(u32 bit)
> +{

[ ... ]

> +	if (bmp_test_and_clear_bit(bit, bitmap))
> +		return -EINVAL;
> +
> +	bmp_set_bit(bit, bitmap);
> +	if (!bmp_test_bit(bit, bitmap))
> +		return -EINVAL;

[Severity: Medium]
Will this test fail spuriously if the BPF loop budget is exhausted during
the atomic operation retries?

The test executes a high number of iterations to intentionally run until
the BPF loop budget (can_loop) is exhausted.

Because the two threads heavily contend on the same bitmap words, cmpxchg
operations in helpers like bmp_set_bit can frequently fail and retry.

If the budget runs out precisely during a retry inside bmp_set_bit, the
helper will exit early without performing the atomic operation. This
function then unconditionally asserts the outcome by checking bmp_test_bit.

Since the bit was never set, this will fail and abort the test with
-EINVAL, rather than cleanly terminating due to budget exhaustion.

> +
> +	bmp_clear_bit(bit, bitmap);
> +	if (bmp_test_bit(bit, bitmap))
> +		return -EINVAL;

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260706181730.21731-1-emil@etsalapatis.com?part=6