Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again)

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  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.