Re: [Threads-devel] v2/shared_mutex bitmask refactoring

Subject: Re: [Threads-devel] v2/shared_mutex bitmask refactoring
From: Fredrik Orderud (forderud_at_[hidden])
Date: 2013-03-26 14:09:58

On Tue, Mar 26, 2013 at 5:57 AM, Vicente J. Botet Escriba <
vicente.botet_at_[hidden]> wrote:

> BTW,
> do you understand why there is a test in (*)?
>
>     void
>     upgrade_mutex::unlock_shared()
>     {
>       boost::lock_guard<mutex_t> _(mut_);
>       count_t num_readers = (state_ & n_readers_) - 1;
>       state_ &= ~n_readers_;
>       state_ |= num_readers;
>       if (state_ & write_entered_)
>       {
>         if (num_readers == 0)
>           gate2_.notify_one();
>       }
>       else
>       {
>         if (num_readers == n_readers_ - 1) // (*)
>           gate1_.notify_one();
>       }
>     }
>

Assuming that gate1_ controls read-locks, then the check at (*) appears to
revive any waiting lock_shared attempts that have previously been blocked
due to the limit of maximum (2^31-1) simultaneous read-locks.

Unless I misunderstand something, then this appears to be an extremely
unlikely situation. Neither pthread/shared_mutex nor win32/shared_mutex
have any similar protection against reaching the maximum limit of
simultaneous read-locks.

Fredrik