Re: [boost] [thread 1.48] Multiple interrupt/timed_join leads to deadlock

Subject: Re: [boost] [thread 1.48] Multiple interrupt/timed_join leads to deadlock
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2012-12-05 12:49:34

On 05/12/2012 18.35, Vicente J. Botet Escriba wrote:
> Le 05/12/12 16:59, Gaetano Mendola a écrit :
>> On 05/12/2012 16.29, Vicente Botet wrote:
>>> Gaetano Mendola-3 wrote
>>>> On 05/12/2012 13.42, Vicente J. Botet Escriba wrote:
>>>>> Le 05/12/12 12:33, Gaetano Mendola a écrit :
>>>>>> On 05/12/2012 09.16, Anthony Williams wrote:
>>>>>>> On 04/12/12 18:32, Gaetano Mendola wrote:
>>>>>>>> Hi all,
>>>>>>>> I was investigating a rare deadlock when issuing an interrupt and
>>>>>>>> a timed_join in parallel. I come out with the the following code
>>>>>>>> showing the behavior.
>>>>>>>>
>>>>>>>> The deadlock is rare so sometime you need to wait a bit.
>>>>>>>>
>>>>>>>> I couldn't try it with boost 1.52 because the code is invalid
>>>>>>>> due the precondition of "thread joinable" when issuing the
>>>>>>>> timed_join.
>>>>>>>
>>>>>>> That's a hint.
>>>>>>>
>>>>>>>> Is the code not valid or a real bug?
>>>>>>>
>>>>>>> The code is invalid: you keep trying to interrupt and join even
>>>>>>> after
>>>>>>> the thread has been joined! Once the thread has been joined, the
>>>>>>> thread
>>>>>>> handle is no longer valid, and you should exit the loop.
>>>>>>
>>>>>> I haven't seen this statement in the documentation.
>>>>>> The loop was meant to exploit exactly this, then you are confirming
>>>>>> that interrupting a joined thread is not valid. How do I safely
>>>>>> interrupt then a thread?
>>>>>> There is no "atomic" check_joinable_then_interrupt, whatching at the
>>>>>> interrupt code it seems that the check is done inside. I'm lost.
>>>>> Boost.Thread and std::thread are designed so that there is only one
>>>>> owner of the thread. That is only one thread can join/interrupt a
>>>>> thread
>>>>> safely.
>>>>
>>>> Unless I have totally missed it the documentation doesn't mention
>>>> anything about thread safety (would that be an hint about it?).
>>>
>>>  From the 1.48 documentation
>>> "Member function timed_join()
>>>
>>> bool timed_join(const system_time& wait_until);
>>>
>>> template<typename TimeDuration>
>>> bool timed_join(TimeDuration const& rel_time);
>>>
>>> Preconditions:
>>>
>>>      this->get_id()!=boost::this_thread::get_id()
>>>
>>> Postconditions:
>>>
>>>      If *this refers to a thread of execution on entry, and timed_join
>>> returns true, that thread of execution has completed, and *this no
>>> longer
>>> refers to any thread of execution. If this call to timed_join returns
>>> false,
>>> *this is unchanged.
>>> "
>>>
>>> Your second call doesn't satisfy the pre-conditions, so that the
>>> outcome of
>>> this second call is undefined.
>>
>> That precondition tests that your are not interrupting yourself
>> doesn't say anything about thread safety. Am I missing something ?
>>
> Hi,
>
> no function is considered been thread-safe until it is stated explicitly.

You are completely right, it was my own fault considering it thread safe.

So even thread_group is not thread safe? The implementation is full 
thread safe. Is that just implementation detail and we have to not rely
on the fact is thread safe?

Regards
Gateano Mendola

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