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

[Mathieu Desnoyers]

----- On Oct 28, 2020, at 5:23 PM, Alexei Starovoitov alexei.starovoitov@xxxxxxxxx wrote:

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

We're working on an updated patchset for those "sleepable tracepoints", and considering
that those are really "tracepoints allowing page faults", I must admit that I am
uncomfortable with the confusion between "sleep" and "fault" in the naming here.

I am tempted to do the following changes:

- Change name from "sleepable tracepoints" to a better suited "tracepoints allowing page faults",
- Use might_fault() rather than might_sleep() in __DO_TRACE(), effectively guaranteeing that all
  probes connecting to a tracepoint which allows page faults can indeed take page faults.
- Change TRACEPOINT_MAYSLEEP into TRACEPOINT_MAYFAULT.

Any objections ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com