linux-kernel - Re: [RFC PATCH 1/6] tracing: introduce sleepable tracepoints

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Message-ID: <20201026224301.gi4bakmj3pov5zyu@ast-mbp.dhcp.thefacebook.com>
Date:   Mon, 26 Oct 2020 15:43:01 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Michael Jeanson <mjeanson@...icios.com>
Cc:     linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        Steven Rostedt <rostedt@...dmis.org>,
        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>,
        Joel Fernandes <joel@...lfernandes.org>, bpf@...r.kernel.org
Subject: Re: [RFC PATCH 1/6] tracing: introduce sleepable tracepoints

On Fri, Oct 23, 2020 at 03:53:47PM -0400, Michael Jeanson wrote:
> -#define __DO_TRACE(tp, proto, args, cond, rcuidle)			\
> +#define __DO_TRACE(tp, proto, args, cond, rcuidle, tp_flags)		\
>  	do {								\
>  		struct tracepoint_func *it_func_ptr;			\
>  		void *it_func;						\
>  		void *__data;						\
>  		int __maybe_unused __idx = 0;				\
> +		bool maysleep = (tp_flags) & TRACEPOINT_MAYSLEEP;	\
>  									\
>  		if (!(cond))						\
>  			return;						\
> @@ -170,8 +178,13 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		/* srcu can't be used from NMI */			\
>  		WARN_ON_ONCE(rcuidle && in_nmi());			\
>  									\
> -		/* keep srcu and sched-rcu usage consistent */		\
> -		preempt_disable_notrace();				\
> +		if (maysleep) {						\
> +			might_sleep();					\

The main purpose of the patch set is to access user memory in tracepoints, right?
In such case I suggest to use stronger might_fault() here.
We used might_sleep() in sleepable bpf and it wasn't enough to catch
a combination where sleepable hook was invoked while mm->mmap_lock was
taken which may cause a deadlock.

> +			rcu_read_lock_trace();				\
> +		} else {						\
> +			/* keep srcu and sched-rcu usage consistent */	\
> +			preempt_disable_notrace();			\
> +		}							\