Re: [Threads-devel] about timed_mutex

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2007-07-02 08:22:52

rick68 <rick68_at_[hidden]> writes:

> On 7/2/07, Anthony Williams <anthony_w.geo_at_[hidden]> wrote:
>>
>> rick68 <rick68_at_[hidden]> writes:
>>
>> > I try timed_mutex whit linux, but it not work.
>>
>> > void timed_mutex::do_lock()
>> > {
>> >    int res = 0;
>> >    res = pthread_mutex_lock(&m_mutex);
>> >    assert(res == 0);
>> >
>> >    while (m_locked)
>> >    {
>> >        res = pthread_cond_wait(&m_condition, &m_mutex);
>> >        assert(res == 0);
>> >    }
>> >
>> >    assert(!m_locked);
>> >    m_locked = true;
>> >
>> >    res = pthread_mutex_unlock(&m_mutex);
>> >    assert(res == 0);
>> > }
>> > All while-loop never running, because of
>> > boost::timed_mutex::scoped_timed_lock will check m_locked is false.
>>
>> Yes. The loops are there so that if the mutex is already locked (m_locked
>> is
>> true), do_lock will wait for the lock to become available. If the mutex is
>> unlocked (m_locked is false) then do_lock doesn't have to wait, so doesn't
>> run
>> the while loop.
>
>
> If I use boost::timed_mutex::scoped_timed_lock to do lock (m_locked is true)
>
> ==<boost/thread/detail/lock.hpp>==
> void boost::detail::thread::scoped_try_lock<boost::timed_mutex>::lock()
> {
>        if (m_locked) throw lock_error();
>        lock_ops<TimedMutex>::lock(m_mutex);
>        m_locked = true;
> }
> ========================
>
> lock will throw lock_error, so I thought maybe to use while-loop will be
> fine.

This is a different m_locked. The m_locked in timed_mutex is the variable
associated with the mutex --- is the mutex locked by any thread? The m_locked
in the scoped_timed_lock member function you quoted above is associated with
the scoped_timed_lock --- does this scoped_timed_lock object own the
associated mutex?

>> So, I change while-loop to do-while-loop:
>> >
>> > ============================================
>> > void timed_mutex::do_lock()
>> > {
>> >    int res = 0;
>> >    res = pthread_mutex_lock(&m_mutex);
>> >    assert(res == 0);
>> >
>> >    do
>> >    {
>> >        res = pthread_cond_wait(&m_condition, &m_mutex);
>> >        assert(res == 0);
>> >    } while (res);
>> >
>> >    assert(!m_locked);
>> >    m_locked = true;
>> >
>> >    res = pthread_mutex_unlock(&m_mutex);
>> >    assert(res == 0);
>> > }
>>
>> This will deadlock! If there is no other thread holding the mutex
>> (m_locked is
>> false) then this will enter the condition wait, but no thread will notify
>> the
>> condition: deadlock.
>
>
> Oh, you are right! :p
> But I don't have any idea let timed_mutex to wait... :(
>
>
>> It's work very well~ :)
>>
>> What exactly are you trying to achieve? If you let us know what you want
>> to do
>> (in your code), then someone might be able to propose a way of achieving
>> that
>> without breaking timed_mutex.
>
>
> I just read about thread code and test it.
>
> Can use condition and mutex to substitute for timed_mutex, but I want to
> know how to use timed_mutex.

timed_mutex is just a mutex, but with timeouts on the locks --- if the mutex
is locked when you try and acquire it, you can pass in a timeout to specify
how long to wait before giving up trying to acquire the lock. Look at
test_timedlock in libs/thread/test/test_mutex.cpp for example usage.

Anthony
-- 
Anthony Williams
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