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

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Message-ID: <20201028212350.jj4mbnbk6hdcbymo@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 28 Oct 2020 14:23:50 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Michael Jeanson <mjeanson@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>, paulmck <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, acme <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, Google" <joel@...lfernandes.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH 1/6] tracing: introduce sleepable tracepoints

On Tue, Oct 27, 2020 at 09:37:08AM -0400, Mathieu Desnoyers wrote:
> 
> ----- On Oct 26, 2020, at 6:43 PM, Alexei Starovoitov alexei.starovoitov@...il.com wrote:
> 
> > 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?
> 
> Yes, exactly.
> 
> > 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.
> 
> Good point! We will do that for the next round.
> 
> By the way, we named this "sleepable" tracepoint (with flag TRACEPOINT_MAYSLEEP),
> but we are open to a better name. Would TRACEPOINT_MAYFAULT be more descriptive ?
> (a "faultable" tracepoint sounds weird though)

bpf kept 'sleepable' as a name. 'faultable' is too misleading.