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Subject: Re: [boost] [thread 1.48] Multiple interrupt/timed_join leads to deadlock
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2012-12-05 15:02:21

On 05/12/2012 19.44, Vicente J. Botet Escriba wrote:
> Le 05/12/12 18:49, Gaetano Mendola a écrit :
>> 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 handle is no longer valid, and you should exit the loop.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.
> How it is thread-safe? Only one thread can call to the thread_group
> functions at a time.

The implementation is straight.
The boost::thread_group keeps a std::list of boost::threads pointers.

the public methods:


are all protecting the internal status and then all the threads in the
list with a common mutex.

It appears completely thread safe to me.

Gaetano Mendola


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