[Libsigcx-main] Re: [sigc] Re: [gtkmm] libsigcx and gtkmm 2.4

[Christer P. <pa...@no...>]

Daniel Elstner wrote:
> 
> Okay, you're (partly) right.  ("Partly" because it's not "locking or
> unlocking": what's needed is unlock in thread A and lock in thread B.)
> I found this in Butenhof:
> 
>         Whatever memory values a thread can see when it unlocks a mutex,
>         either directly or by waiting on a condition variable, can also
>         be seen by any thread that later locks the same mutex.  Again,
>         data written after the mutex is unlocked may not necessarily be
>         seen by the thread that locks the mutex, even if the write
>         occurs before the lock.
> 
> In other words, the sequence
> 
>         pthread_mutex_lock(mutex);
>         pthread_mutex_unlock(mutex);
> 
> issues a memory barrier instruction on the unlock.  The other thread
> that wants to read the data still has to lock the same mutex though.
> 

A memory barrier, or synchronize, instruction is issued both on lock and 
unlock and also in a bunch of other thread related functions. Of course 
all threads need to agree on which mutex protects memory location X, 
that's how they make sure they doesn't execute a region of code that 
access memory location X simultaneously. Not because only certain memory 
locations are syncronized then the mutex is locked/unlocked.

Having said that, is there any place in mine or Martins code where you 
believe that this rule isn't followed, except as a side effect of 
passing objects that contain internal references?


This is what IEEE Std 1003.1-2004 has to say about memory 
synchronization requirements:

  4.10 Memory Synchronization

Applications shall ensure that access to any memory location by more 
than one thread of control (threads or processes) is restricted such 
that no thread of control can read or modify a memory location while 
another thread of control may be modifying it. Such access is restricted 
using functions that synchronize thread execution and also synchronize 
memory with respect to other threads. The following functions 
synchronize memory with respect to other threads:

...
pthread_mutex_lock()
...
pthread_mutex_unlock()
...

--
Christer Palm