Unsynchronized int access from threads
Paul Backus
snarwin at gmail.com
Thu Jun 18 16:55:26 UTC 2020
On Thursday, 18 June 2020 at 16:42:15 UTC, H. S. Teoh wrote:
> Suppose I have an int[] which may contain some zeroes, and 2
> functions, one scans the array for zeroes but does not modify
> it, and the other modifies array elements but never introduces
> new zeroes (though it may write a 0 to an existing 0). Is it
> thread-safe to run the two functions in parallel without any
> synchronization between them?
>
> I.e., will the first function always see zeroes where there are
> zeroes, in spite of the 2nd function writing to the array
> simultaneously? Are there any hardware situations where the
> 1st function may read a non-zero value if the 2nd function is
> simultaneously overwriting an existing zero with another zero?
> Or a situation where the 1st function may read a zero if the
> 2nd function is simultaneously overwriting a non-zero value
> with another non-zero value?
>
> Or am I playing with fire here?
>
>
> T
As long as reads and writes of `int` are atomic on the platform
in question (which they probably are) you should be ok.
Personally, I would use core.atomic.atomic{Load,Store} just to be
safe; they should optimize themselves away if they're not needed.
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