Re: [PATCH v3 1/2] ftrace: disable preemption between ftrace_test_recursion_trylock/unlock()

[王贇]

On 2021/10/15 下午3:28, Petr Mladek wrote:
> On Fri 2021-10-15 11:13:08, 王贇 wrote:
>>
>>
>> On 2021/10/14 下午11:14, Petr Mladek wrote:
>> [snip]
>>>> -	return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
>>>> +	int bit;
>>>> +
>>>> +	bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
>>>> +	/*
>>>> +	 * The zero bit indicate we are nested
>>>> +	 * in another trylock(), which means the
>>>> +	 * preemption already disabled.
>>>> +	 */
>>>> +	if (bit > 0)
>>>> +		preempt_disable_notrace();
>>>
>>> Is this safe? The preemption is disabled only when
>>> trace_test_and_set_recursion() was called by ftrace_test_recursion_trylock().
>>>
>>> We must either always disable the preemtion when bit >= 0.
>>> Or we have to disable the preemtion already in
>>> trace_test_and_set_recursion().
>>
>> Internal calling of trace_test_and_set_recursion() will disable preemption
>> on succeed, it should be safe.
> 
> trace_test_and_set_recursion() does _not_ disable preemtion!
> It works only because all callers disable the preemption.

Yup.

> 
> It means that the comment is wrong. It is not guarantted that the
> preemption will be disabled. It works only by chance.
> 
> 
>> We can also consider move the logical into trace_test_and_set_recursion()
>> and trace_clear_recursion(), but I'm not very sure about that... ftrace
>> internally already make sure preemption disabled
> 
> How? Because it calls trace_test_and_set_recursion() via the trylock() API?

I mean currently all the direct caller of trace_test_and_set_recursion()
have disabled the preemption as you mentioned above, but yes if anyone later
write some kernel code to call trace_test_and_set_recursion() without
disabling preemption after, then promise broken.

> 
> 
>> , what uncovered is those users who call API trylock/unlock, isn't
>> it?
> 
> And this is exactly the problem. trace_test_and_set_recursion() is in
> a public header. Anyone could use it. And if anyone uses it in the
> future without the trylock() and does not disable the preemtion
> explicitely then we are lost again.
> 
> And it is even more dangerous. The original code disabled the
> preemtion on various layers. As a result, the preemtion was disabled
> several times for sure. It means that the deeper layers were
> always on the safe side.
> 
> With this patch, if the first trace_test_and_set_recursion() caller
> does not disable preemtion then trylock() will not disable it either
> and the entire code is procceed with preemtion enabled.

Yes, what confusing me at first is that I think people who call
trace_test_and_set_recursion() without trylock() can only be a
ftrace hacker, not a user, but in case if anyone can use it without
respect to preemption stuff, then I think the logical should be inside
trace_test_and_set_recursion() rather than trylock().

> 
> 
>>> Finally, the comment confused me a lot. The difference between nesting and
>>> recursion is far from clear. And the code is tricky liky like hell :-)
>>> I propose to add some comments, see below for a proposal.
>> The comments do confusing, I'll make it something like:
>>
>> The zero bit indicate trace recursion happened, whatever
>> the recursively call was made by ftrace handler or ftrace
>> itself, the preemption already disabled.
> 
> I am sorry but it is still confusing. We need to find a better way
> how to clearly explain the difference between the safe and
> unsafe recursion.
> 
> My understanding is that the recursion is:
> 
>   + "unsafe" when the trace code recursively enters the same trace point.
> 
>   + "safe" when ftrace_test_recursion_trylock() is called recursivelly
>     while still processing the same trace entry.

Maybe take some example would be easier to understand...

Currently there are two way of using ftrace_test_recursion_trylock(),
one with TRACE_FTRACE_XXX we mark as A, one with TRACE_LIST_XXX we mark
as B, then:

A followed by B on same context got bit > 0
B followed by A on any context got bit 0
A followed by A on same context got bit > 0
A followed by A followed by A on same context got bit -1
B followed by B on same context got bit > 0
B followed by B followed by B on same context got bit -1

If we get rid of the TRACE_TRANSITION_BIT which allowed recursion for
onetime, then it would be:

A followed by B on same context got bit > 0
B followed by A on any context got bit 0
A followed by A on same context got bit -1
B followed by B on same context got bit -1

So as long as no continuously AAA it's safe?

> 
>>>> +
>>>> +	return bit;
>>>>  }
>>>>  /**
>>>> @@ -222,9 +233,13 @@ static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
>>>>   * @bit: The return of a successful ftrace_test_recursion_trylock()
>>>>   *
>>>>   * This is used at the end of a ftrace callback.
>>>> + *
>>>> + * Preemption will be enabled (if it was previously enabled).
>>>>   */
>>>>  static __always_inline void ftrace_test_recursion_unlock(int bit)
>>>>  {
>>>> +	if (bit)
>>>
>>> This is not symetric with trylock(). It should be:
>>>
>>> 	if (bit > 0)
>>>
>>> Anyway, trace_clear_recursion() quiently ignores bit != 0
>>
>> Yes, bit == 0 should not happen in here.
> 
> Yes, it "should" not happen. My point is that we could make the API
> more safe. We could do the right thing when
> ftrace_test_recursion_unlock() is called with negative @bit.
> Ideally, we should also warn about the mis-use.

Agree with a WARN here on bit 0.

> 
> 
> Anyway, let's wait for Steven. It seems that he found another problem
> with the API that should be solved first. The fix will probably
> also help to better understand the "safe" vs "unsafe" recursion.

Cool~

Regards,
Michael Wang

> 
> Best Regards,
> Petr
>