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

[Mathieu Desnoyers]

----- 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)

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