Re: Object-oriented multithreading

From:
Pete Becker <petebecker@acm.org>
Newsgroups:
comp.lang.c++.moderated
Date:
6 Sep 2006 10:49:45 -0400
Message-ID:
<srKdnVu0vdZxImPZnZ2dnUVZ_ridnZ2d@giganews.com>
Alan McKenney wrote:

One of the big issues is synchronization, especially
of memory reads and writes. For example, if we have

int shared_value = 0;
Mutex shared_value_mutex;
...

void thread_a() {
   shared_value_mutex.lock();
   shared_value += 10;
   shared_value_mutex.unlock();
  }
void thread_b() {
   shared_value_mutex.lock();
   shared_value += 20;
   shared_value_mutex.unlock();
  }

and we assume that thread_b() starts before thread_a()
finishes, it is conceivable that thread_a() will release
the mutex, and thread_b() will get it, before thread_a's
update of shared_value propagates to the processor running
thread_b.


Maybe I'm missing the point. If the mutex is properly implemented, this
isn't a problem. Unlocking does a release, and locking does an acquire.
The result is that any values written by the unlocking thread are
visible to the locking thread.

--

  -- Pete

Author of "The Standard C++ Library Extensions: a Tutorial and Reference."
For more information about this book, see www.petebecker.com/tr1book.

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