Race conditions

A race condition or race hazard is the behavior of an electronic, software, or other system where the output is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when events do not happen in the order the programmer intended. The term originates with the idea of two signals racing each other to influence the output first.

Race conditions can occur in electronics systems, especially logic circuits, and in computer software, especially multithreaded or distributed programs.

A typical example of a race condition may occur in a system of logic gates where inputs vary. If a given output depends on the state of the inputs it may only be defined for steady-state signals. As the inputs change state a small delay will occur before the output changes due to the physical nature of the electronic system. 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 a logic signal A on one input and its negation, NOT A, on another input. In theory the output (A AND NOT A) should never be true. If, however, changes in the value of A take longer to propagate to the second input than the first when A 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.

Design techniques such as Karnaugh maps encourage designers to recognize and eliminate race conditions before they cause problems. Often logic redundancy can be added to eliminate some kinds of races.

As well as these problems, some logic elements can enter metastable states, which create further problems for circuit designers.

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