![]() Operations upon shared states are done in critical sections that must be mutually exclusive. Critical race conditions often happen when the processes or threads depend on some shared state. ![]() A race can also occur between two programs, resulting in security issues (see below.)Ĭritical race conditions cause invalid execution and software bugs. If the multiple code paths take a different amount of time than expected, they can finish in a different order than expected, which can cause software bugs due to unanticipated behavior. Often logic redundancy can be added to eliminate some kinds of races.Īs well as these problems, some logic elements can enter metastable states, which create further problems for circuit designers.Ī race condition can arise in software when a computer program has multiple code paths that are executing at the same time. Sometimes they are cured using inductive delay line elements to effectively increase the time duration of an input signal.ĭesign techniques such as Karnaugh maps encourage designers to recognize and eliminate race conditions before they cause problems. It can be eliminated by using no more than two levels of gating.Īn essential race condition occurs when an input has two transitions in less than the total feedback propagation time. They are due to interaction between gates. Static, dynamic, and essential forms Ī static race condition occurs when a signal and its complement are combined.Ī dynamic race condition occurs when it results in multiple transitions when only one is intended. If all the bits of the counter do not change exactly simultaneously, there will be intermediate patterns that can trigger false matches.Ī 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.Ī non-critical race condition occurs when the order in which internal variables are changed does not determine the eventual state that the state machine will end up in. Ī practical example of a race condition can occur when logic circuitry is used to detect certain outputs of a counter. Output = A ∧ 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. 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).Ĭonsider, for example, a two-input AND gate fed with the following logic: The output may, for a brief period, change to an unwanted state before settling back to the designed state. The inputs to the gate can change at slightly different times in response to a change in the source signal. Race conditions can occur especially in logic circuits, multithreaded, or distributed software programs.Ī typical example of a race condition may occur when a logic gate combines signals that have traveled along different paths from the same source. Huffman's doctoral thesis "The synthesis of sequential switching circuits". The term race condition was already in use by 1954, for example in David A. It becomes a bug when one or more of the possible behaviors is undesirable. ![]() When the input value A changes from low to high, the circuit outputs a short spike of duration (∆ t 1 + ∆ t 2) − ∆ t 2 = ∆ t 1.Ī 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. Here, ∆ t 1 and ∆ t 2 represent the propagation delays of the logic elements. For the academic movement, see Critical race theory. ![]()
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