Re: [PATCH bpf-next v5 2/3] perf: Add atomic operation in get_recursion_context

Peter Zijlstra
2025-11-10T08:52:20Z

On Mon, Nov 10, 2025 at 12:35:58AM +0800, Tao Chen wrote:
> From BPF side, preemption usually is enabled. Yonghong said, it is
> possible that both tasks (at process level) may reach right before
> "recursion[rctx]++;". In such cases, both tasks will be able to get
> buffer and this is not right.
>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
> ---

Nope, this function really is meant to be used with preemption disabled.
If BPF doesn't abide, fix that.

> kernel/events/internal.h | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index d9cc5708309..684bde972ba 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -214,12 +214,9 @@ static inline int get_recursion_context(u8 *recursion)
> {
> unsigned char rctx = interrupt_context_level();
>
> - if (recursion[rctx])
> + if (cmpxchg(&recursion[rctx], 0, 1) != 0)
> return -1;
>
> - recursion[rctx]++;
> - barrier();
> -
> return rctx;
> }
>
> --
> 2.48.1
>