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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 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 handle is no longer valid, and you should exit the loop.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?

Gateano Mendola


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