linux-kernel - Re: [PATCH v4 16/27] tracing: Remove regular RCU context for

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Message-ID: <20200306104335.GF3348@worktop.programming.kicks-ass.net>
Date:   Fri, 6 Mar 2020 11:43:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org
Cc:     mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v4 16/27] tracing: Remove regular RCU context for
 _rcuidle tracepoints (again)

On Fri, Feb 21, 2020 at 02:34:32PM +0100, Peter Zijlstra wrote:
> Effectively revert commit 865e63b04e9b2 ("tracing: Add back in
> rcu_irq_enter/exit_irqson() for rcuidle tracepoints") now that we've
> taught perf how to deal with not having an RCU context provided.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  include/linux/tracepoint.h |    8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -179,10 +179,8 @@ static inline struct tracepoint *tracepo
>  		 * For rcuidle callers, use srcu since sched-rcu	\
>  		 * doesn't work from the idle path.			\
>  		 */							\
> -		if (rcuidle) {						\
> +		if (rcuidle)						\
>  			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
> -			rcu_irq_enter_irqsave();			\
> -		}							\
>  									\
>  		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
>  									\
> @@ -194,10 +192,8 @@ static inline struct tracepoint *tracepo
>  			} while ((++it_func_ptr)->func);		\
>  		}							\
>  									\
> -		if (rcuidle) {						\
> -			rcu_irq_exit_irqsave();				\
> +		if (rcuidle)						\
>  			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
> -		}							\
>  									\
>  		preempt_enable_notrace();				\
>  	} while (0)

So what happens when BPF registers for these tracepoints? BPF very much
wants RCU on AFAIU.