linux-kernel - Re: [RFC PATCH v3 1/5] tracing: Introduce faultable tracepoints (v3)

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Date:   Mon, 2 Oct 2023 19:10:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel@...r.kernel.org,
        Michael Jeanson <mjeanson@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, bpf@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC PATCH v3 1/5] tracing: Introduce faultable tracepoints
 (v3)

On Mon,  2 Oct 2023 16:25:27 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> @@ -202,8 +198,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (WARN_ON_ONCE(RCUIDLE_COND(rcuidle)))		\
>  			return;						\
>  									\
> -		/* keep srcu and sched-rcu usage consistent */		\
> -		preempt_disable_notrace();				\
> +		if (mayfault) {						\
> +			rcu_read_lock_trace();				\

I thought rcu_trace was for the case that a task can not voluntarily call
schedule. If this tracepoint tries to read user space memory that isn't
paged in, and faults, can't the faulting logic call schedule and break this
requirement?

-- Steve


> +		} else {						\
> +			/* keep srcu and sched-rcu usage consistent */	\
> +			preempt_disable_notrace();			\
> +		}							\
>  									\
>  		/*							\
>  		 * For rcuidle callers, use srcu since sched-rcu	\