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
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