Re: [PATCH bpf v3 2/2] selftests/bpf: verify preemptible uprobes avoid private stack

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Spurious test failure (flakiness) due to insufficiently small timeout limit in bpf_loop.
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--- Patch [2]: [PATCH bpf v3 2/2] selftests/bpf: verify preemptible uprobes avoid private stack ---
commit c17909e48f5484c8e9bf3341a1fc87b98ad29988
Author: Christian Simon <[email protected]>

selftests/bpf: verify preemptible uprobes avoid private stack
    
Pin two threads to one CPU and overlap invocations of the same uprobe
program. Verify that both invocations execute and that the first
invocation's stack frame is not corrupted by the second.

> diff --git a/tools/testing/selftests/bpf/progs/uprobe_sleepable_stack.c b/tools/testing/selftests/bpf/progs/uprobe_sleepable_stack.c
> index 0000000000000..07e0ff5487208
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/uprobe_sleepable_stack.c
[ ... ]
> +static __always_inline int run_stack_test(volatile __u64 *stack)
> +{
[ ... ]
> +	ready = 1;
> +	bpf_loop(1 << 23, wait_for_release, NULL, 0);
> +	if (!release)
> +		loop_exhausted = 1;

[Severity: Medium]
Could this iteration limit cause spurious test failures under load?

In a heavily loaded CI environment, the main thread's usleep(1000) can be
delayed significantly (for example, if timer granularity is 10ms or due to
hypervisor latency). On a fast CPU, 8.38M iterations of this trivial callback
might complete in just a few milliseconds, exhausting the loop before the
main thread wakes up and sets release.

If this happens, loop_exhausted is set to 1 and the selftest fails.
Is it possible to safely extend this wait time by wrapping the bpf_loop call
in an outer bounded loop?

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=2