Re: [LAD] Atomic Operations

From: Victor Lazzarini <Victor.Lazzarini@email-addr-hidden>
Date: Sat Dec 12 2009 - 20:43:26 EET

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@email-addr-hidden
> http://lists.linuxaudio.org/mailman/listinfo/linux-audio-dev

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Received on Sun Dec 13 00:15:01 2009

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