Re: [boost] Re: [Boost.Thread] mutex Win32 implementation changes

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-19 11:41:01

Alexander Terekhov wrote:
>
> With respect to swap_based_mutex_for_windows thing (without
> pimpl-and-never-destroyed-impl), the problem is that it can go boom
> in unlock().
>
>   void lock() throw() {
>     if (m_lock_status.swap(1, msync::acq))
>       while (m_lock_status.swap(-1, msync::acq))
>         m_retry_event.wait();
>   }
>
>   void unlock() throw() {
>     if (m_lock_status.swap(0, msync::rel) < 0)
>       m_retry_event.set();
>   }
>
> Scenario...
>
>   Given: mutex protected refcounting. Two threads, A and B.
>
>      t0: thread A locks the mutex and decrements refcount to 2;
>
>      t1: thread B does m_lock_status.swap(1, msync::acq) on the
>          fast path and sees 1;
>
>      t2: thread A unlocks the mutex (doesn't enter slow path);
>
>      t3: thread B does mutex m_lock_status.swap(-1, msync::acq),
>          locks the mutex, decrements refcount to 1, does
>          m_lock_status.swap(0, msync::rel) and enters slow path
>          in unlock();

    SetEvent( e_ );

>      t4: thread A locks the mutex, decrements refcount to 0,
>          unlocks the mutex, and destroys it (including event);

    CloseHandle( e_ );

>      t5: thread B goes BOOM.

Does it? The Win32 kernel is pretty resilient. ;-) We'll have to try it, I
guess.