[M3devel] race conditions in lock-free code...

[Jay K]

We have race conditions, like this, m3core/src/Text.m3:


VAR fromCharCache := ARRAY CHAR OF T {NIL, ..}; (* 1-char texts *)


PROCEDURE FromChar (c: CHAR): T =
  VAR buf: ARRAY [0..0] OF CHAR;
  BEGIN
    IF fromCharCache [c] = NIL THEN
      buf [0] := c;
      fromCharCache[c] := Text8.New (buf);
    END;
    RETURN fromCharCache [c]
  END FromChar;


It should be:


PROCEDURE FromChar (c: CHAR): T =

  VAR buf: ARRAY [0..0] OF CHAR;

  BEGIN

    IF fromCharCache [c] = NIL THEN

      buf [0] := c;

      WITH a = Text8.New (buf) DO
        MemoryBarrier();
        fromCharCache[c] := a;
     END;

    END;

    RETURN fromCharCache [c]

  END FromChar;



to ensure that all of Text8.New() finishes before the assignment to fromCharCache[c] is made.


Can the compiler somehow catch these?
I fear they are a small epidemic.
For a long time people didn't realize where all the compiler and processor could reorder.


Do we have the right constructs by now to fix them?
I think we do.


 - Jay
 		 	   		  
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