Re: [RESEND PATCH v2] trace: prevent preemption in perf_ftrace_function_call()

[Peter Zijlstra]

On Mon, Oct 11, 2021 at 10:32:46AM +0200, Peter Zijlstra wrote:
> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index a9f9c5714e65..ca12e2d8e060 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -214,7 +214,14 @@ static __always_inline void trace_clear_recursion(int bit)
>  static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
>  							 unsigned long parent_ip)
>  {
> -	return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> +	bool ret;
> +
> +	preempt_disable_notrace();
> +	ret = trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> +	if (!ret)
> +		preempt_enable_notrace();
> +
> +	return ret;
>  }
>  
>  /**

Oh, I might've gotten that wrong, I assumed regular trylock semantics,
but it doesn't look like that's right.