is +=1 thread safe

[Gary Herron]

AlFire wrote:
> Hi,
>
> I have a piece of software which uses threads in very massive way - 
> like hundreds of  them generated every second.
>
> there is also a piece of code which maintains the number of 
> outstanding threads, simply
>
> counter+=1 is executed when before starting the thread and counter-=1 
> after it finishes.
>
> all is very simple and by the end of the program life I expect the 
> counter to zero out.
>
> however I am getting values -1, -2, 1 ,2 ,3 and quite often 0 as 
> expected.
>
> I guarded those statement with Lock.{acquire,release} and now it 
> always returns 0.
>
>
> But I still can not believe that +=1 is not a thread safe operation.
>
>
> Any clue?
>
Of course it's not thread safe.   For the same reason and more basic, 
even the expression i++ is not thread safe in C++.

Any such calculation, on modern processors, requires three operations: 
  retrieve value of i into a register,
  increment the register
  write the value into i.

If a thread is interrupted anywhere within that sequence, and another 
thread access i, you have a conflict.  (And indeed, hardware interrupts 
can occur between any two instructions.)

Gary Herron