[LAD] Atomic Operations

[Victor Lazzarini]

This is very interesting, thanks. Learned a new thing today. I was  
also not aware of sched_setaffinity()
this is actually something I always wondered existed, but had never  
seen before.
Thanks a lot again

Victor

On 12 Dec 2009, at 17:47, Gabriel M. Beddingfield wrote:

>
> Recently on this list, Paul referred to an "atomic integer swap" and  
> an "atomic pointer swap."  This was a new concept to me (and  
> possibly others), and this e-mail shares what I've learned.
>
> If you access a variable from multiple threads -- even a built-in  
> variable like 'int', it is important to control access to the  
> variable with a mutex, semaphore, or an atomic operation.
>
> Alexander Sandler, on his blog, wrote a couple of good articles on  
> the subject:
>
>    "Do you need a mutex to protect an int?"
>    http://www.alexonlinux.com/do-you-need-mutex-to-protect-int
>
>    "Multithreaded simple data type access and atomic
>    variables"
>    http://www.alexonlinux.com/multithreaded-simple-data-type-access-and-atomic-variables
>
> The first article contains code that calculates a wrong answer on  
> multiprocessor machines.  I've attached a similar example that will  
> even fail on a single-processor machine.
>
> There is a wealth of reading material on using Mutexes and  
> Semaphores.  However, information on atomic operations appears to be  
> sparse and hard-to-follow.  So, here's what I've found:
>
>    + At the moment, there is no built-in support in
>      C/C++ for atomic operations.  You will need to use
>      a library, compiler extension, or write your own
>      in assembly code.
>
>    + The GCC compiler has the built-in __sync_*()
>      functions[1] that provide atomic operations.
>      Note that the attached example is using this.
>
>    + glib provides the g_atomic_*() functions[2].
>
>    + Qt 4 has the q_atomic_*() functions.[3]  While
>      they are accessible, they are /not/ a part of
>      their stable, public API.
>
>    + The next version of ISO C++ (code name c++0x)
>      is expected to have support for atomic operations
>      (E.g. the std::atomic<T> template) and memory
>      barriers.  It may even require that all built-in
>      types be atomic.
>
>    + In the x86 instruction set, these are usually
>      implemented using the 'LOCK' instruction prefix.[5]
>
> When using atomic operations, perhaps the best advice I found is  
> near the end of Sandler's second article:
>
>    "When using atomic variables, some extra
>    precautions have to be taken....  There is nothing
>    that prevents you from incrementing value of the
>    atomic variable with __sync_fetch_and_add() as I
>    just demonstrated and later in the code doing same
>    thing with regular ++ operator.
>
>    "To address this problem, I strongly suggest
>    wrapping around atomic functions and variables with
>    either ADT in C or C++ class."[4]
>
> Peace,
> Gabriel
>
> [1] http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
> [2] http://www.gtk.org/api/2.6/glib/glib-Atomic-Operations.html
> [3] http://doc.trolltech.com/4.3/atomic-operations.html
>    See also the Qt header file QtCore/qatomic_i386.h, and
>    its brothers.
> [4] http://www.alexonlinux.com/multithreaded-simple-data-type-access-and-atomic-variables#precautions
> [5] http://siyobik.info/index.php? 
> module 
> = 
> x86 
> &id=159<atomic_fail.c>_______________________________________________
> Linux-audio-dev mailing list
> Linux-audio-dev at lists.linuxaudio.org
> http://lists.linuxaudio.org/mailman/listinfo/linux-audio-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.linuxaudio.org/pipermail/linux-audio-dev/attachments/20091212/a7a20977/attachment.html>