From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-08-25T15:27:01+09:00
Subject: Re: How did this variable become negative?

On Mon, Aug 25, 2003 at 10:54:46AM +0900, Philip Mak wrote:
> So that means @@num_sending must have become -1 somehow, in order for
> a ZeroDivisionError to occur.
> 
> But @@num_sending is initialized to 0, and this is the only place in
> the code where it is modified:
> 
>         begin
>                 @@num_sending += 1
>                 yield limit
>         ensure
>                 @@num_sending -= 1
>         end
> 
> So how could @@num_sending have become -1? It can only be decremented
> after being incremented. Could the fact that this class is used by
> multiple threads simultaneously have something to do with it? But I
> still don't see how it could happen...

As you point out, your code is not threads-safe

Consider the following scenario:
 * 2 threads, called A and B
 * thread A is the first to enter that section of code. In order to do 
   @@num_sending += 1, it first grabs @@num_sending (which is 0) and
   then adds 1 (ie. it's not an atomic operation). Imagine that just
   after A takes the value of @@num_sending, B does the same. You'll
   then have the following

    A                           B
	take @@num_sending => 0
                                take @@num_sending => 0
    add 1
    store 1 in @@num_sending
    (@@num_sending = 1 here)    add 1
                                store 1 in @@num_sending
								(@@num_sending = 1 here)

Now, the -= 1 operations could happen "atomically" (no thread
switching in the middle), then

	take value => 1
	substract 1
	store (0)
								take value => 0
								substract 1
								store -1
								
The solution is changing your code or using a mutex.

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