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Date:   Tue, 27 Oct 2020 09:37:08 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Michael Jeanson <mjeanson@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Yonghong Song <yhs@fb.com>, paulmck <paulmck@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, acme <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        bpf <bpf@vger.kernel.org>
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Subject: Re: [RFC PATCH 1/6] tracing: introduce sleepable tracepoints
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Thread-Topic: tracing: introduce sleepable tracepoints
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----- On Oct 26, 2020, at 6:43 PM, Alexei Starovoitov alexei.starovoitov@gmail.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)

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