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

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Message-ID: <20251110085210.GV3245006@noisy.programming.kicks-ass.net>
Date: Mon, 10 Nov 2025 09:52:10 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
	mark.rutland@....com, alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH bpf-next v5 2/3] perf: Add atomic operation in
 get_recursion_context

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@...ux.dev>
> Signed-off-by: Tao Chen <chen.dylane@...ux.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
>