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Subject: Re: [boost] [thread 1.48] Multiple interrupt/timed_join leads to deadlock
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2012-12-06 19:23:28

On 06/12/2012 23.31, Vicente J. Botet Escriba wrote:
> Le 05/12/12 21:02, Gaetano Mendola a écrit :
>> 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 handle is no longer valid, and you should exit the loop.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:
>> create_thread
>> add_thread
>> remove_thread
>> join_all
>> interrupt_all
>> size
>> 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.
> You are right. I didn't see the implementation. Anthony, I don't know
> what was the intent of the initial design. Do you have an idea?
> What do you suggest? update the documentation?

Even the implementation can be improved, even if it's thread safe it
doesn't avoid to issue a join to an already joined thread, or for the
matter to interrupt a thread already joined/interrupted.

I also believe the create_thread returning a boost::thread pointers is
very dangerous, indeed doing so the boost::thread_group doesn't have
anymore the ownership of it, it would be better if create_thread would
have returned a thread id instead.

Sure the programmer can shoot himself in the foot in other more
spectacular way but at least having a thread group interface obbeying
the principle of least astonishment can be a plus.

Gaetano Mendola


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