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

[Joel Fernandes]

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@xxxxxxxxx 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)

What about keeping it might_sleep() here and then adding might_fault() in the
probe handler? Since the probe handler knows that it may cause page fault, it
could itself make sure about it.

One more thought: Should we make _all_ tracepoints sleepable, and then move
the preempt_disable() bit to the probe handler as needed? That could simplify
the tracepoint API as well. Steven said before that whoever registers probes
knows what they are doing so I am ok with that.

No strong feelings one way or the other, for either of these though.

thanks,

 - Joel

> 
> Thanks,
> 
> Mathieu
> 
> > 
> >> +			rcu_read_lock_trace();				\
> >> +		} else {						\
> >> +			/* keep srcu and sched-rcu usage consistent */	\
> >> +			preempt_disable_notrace();			\
> > > +		}							\
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com