From: Robert Klemme <bob.news@...>
Date: 2006-03-02T19:43:38+09:00
Subject: Re: Newby question: Is += atomic for integers?

Minkoo Seo wrote:
> rj-cole wrote:
>> Can the following program produce counts that are wrong?
>>
>> class Test
>>   attr_accessor :count
>>   def initialize
>>     @count = 0
>>   end
>> end
>>
>> $test = Test.new
>>
>> def my_function_called_by_many_threads
>>   ...
>>   $test.count += 1
>>   ...
>> end
>
> Yes.
>
> There's two problem.
>
> (1) Even though an operation seems to be atomic, say, "a+1", it might
> not be atomic in the machine level. Such an example is operations on
> double, and class references. In some cases, assignment of double
> variable might be done two steps.
>
> (2) Especially if $test were shared by multiple threads which runs on
> SMP, though current version of ruby core does not support it - I hope
> it will - , you have to use mutex(synchronize). Otherwise, different
> threads may see different values because each processor uses own
> cache. This rule even applies to Java, C++, etc.
>
> Simple and best solution to these problems is to use synchronization
> whenever needed.

I strongly support that!  Always do proper synchronization even if a non
synchronized version seems to work.  It's likely to break as soon as the
code runs on another platform (e.g. Java VM).  These bugs are hard to find
and also there is a documentation advantage that comes with proper MT
design.

Kind regards

    robert