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Concept
Race condition
Summary
A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of the possible behaviors is undesirable.
The term race condition was already in use by 1954, for example in David A. Huffman's doctoral thesis "The synthesis of sequential switching circuits".
Race conditions can occur especially in logic circuits, multithreaded, or distributed software programs.
A typical example of a race condition may occur when a logic gate combines signals that have traveled along different paths from the same source. The inputs to the gate can change at slightly different times in response to a change in the source signal. The output may, for a brief period, change to an unwanted state before settling back to the designed state. Certain systems can tolerate such glitches but if this output functions as a clock signal for further systems that contain memory, for example, the system can rapidly depart from its designed behaviour (in effect, the temporary glitch becomes a permanent glitch).
Consider, for example, a two-input AND gate fed with the following logic: A logic signal on one input and its negation, (the ¬ is a boolean negation), on another input in theory never output a true value: . If, however, changes in the value of take longer to propagate to the second input than the first when changes from false to true then a brief period will ensue during which both inputs are true, and so the gate's output will also be true.
A practical example of a race condition can occur when logic circuitry is used to detect certain outputs of a counter. If all the bits of the counter do not change exactly simultaneously, there will be intermediate patterns that can trigger false matches.
A critical race condition occurs when the order in which internal variables are changed determines the eventual state that the state machine will end up in.
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