Subj : Re: Concurrent write undefined?
To   : comp.programming.threads
From : Joe Seigh
Date : Mon May 23 2005 07:40 am

On Mon, 23 May 2005 07:33:01 +0200, Jonas Latt <Jonas.Latt@cui.unige.ch> wrote:

> Dear all
>
> I have a question on the memory model underlying the pthread library (I
> am programming in C++ with pthreads on a Linux platform).
>
> In the code I am concerned about, several threads write concurrently to
> the same shared variable (I am implementing the PRAM version of a
> quicksort. In this model, all threads write their ID to the same
> location, in order to determine in a random way the next pivot). My
> question is: do I need to protect this shared variable by a Mutex? Or is
> the result undefined if I don't? I am of course aware that the result is
> undefined in the sense that it is unpredictable which thread wins the
> race and writes the number which will finally be contained in the shared
> variable; this is actually a desired side-effect of the algorithm. But
> do I at least have the guarantee that the operation results in that a
> well-defined number (one of those proposed by the threads) is written to
> the shared variable, or do I have completely undefined behavior?
>
> When I implement it, the algorithm works well without Mutex. But I doubt
> that what I am doing is portable or even reproducible. Can anybody give
> me a hint on this? Does the answer to this kind of questions depend on
> the thread library? Or the programming language? Or the hardware? I
> would also be very grateful for a pointer to some literature on these
> issues.

int is probably atomic.  Theres some atomic types defined in one of the
Linux headers somewhere, atomic.h IIRC.   The atomicity thing for C/C++
isn't documented anywhere, it's an article of faith or some sort of
secret knowlege shared only be a few Linux Illuminati.

More importantly is do the threads all have to see the same value being
set?  If so then you need something like pthread_once or a mutex depending
on whether the variable is set more than once.  If the latter, you'll need
some additional information to keep track of what instance you're using.



-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.