RACE -FREE STATE ASSIGNMENT
Once a reduced flow table has been derived for an asynchronous sequential circuit, the next step in the design is to assign binary variables to each stable state. This assignment results in the transformation of the flow table into its equivalent transition table. The primary objective in choosing a proper binary state assignment is the prevention of critical races. Critical races can be avoided by making a binary state assignment in such a way that only one variable changes at any given time when a state transition occurs in the flow table.
ü Three-Row Flow-Table Example
Fig: Three row flow table example
To avoid critical races, we must find a binary state assignment such that only one binary variable changes during each state transition. An attempt to find such an assignment is shown in the transition diagram. State a is assigned binary 00, and state c is assigned binary 11. This assignment will ca use a critical race during the transition from a to c because there are two changes in the binary state variables and the transition from a to c may occur directly or pass through b. Note that the transition from c to a also ca uses a race condition, but it is noncritical because the transition does not pass through other states.
A race-free assignment can be obtained if we add an extra row to the flow table. The use of a fourth row does not increase the number of binary state variables, but it allows the formation of cycles between two stable states.
The transition table corresponding to the flow table with the indicated binary state assignment is shown in Fig. The two dashes in row d represent unspecified states that can be considered don't-care conditions. However, care must be taken not to assign 10 to these squares, in order to avoid the possibility of an unwanted stable state being established in the fourth row.
ü Four-Row Flow-Table Example
A flow table with four rows requires a minimum of two state variables. Although a race-free assignment is sometimes possible with only two binary state variables, in many cases the requirement of extra rows to avoid critical races will dictate the use of three binary state variables
Fig: Four-row flow-table example
The following figure shows a state assignment map that is suitable for any four-row flow table. States a, b, c and d are the original states and e, f and g are extra states. The transition from a to d must be directed through the extra state e to produce a cycle so that only one binary variable changes at a time. Similarly, the transition from c to a is directed through g and the transition from d to c goes through f. By using the assignment given by the map, the four-row table can be expanded to a seven-row table that is free of critical races.
Fig: Choosing extra rows for the flow table
Note that although the flow table has seven rows there are only four stable states. The uncircled states in the three extra rows are there merely to provide a race-free transition between the stable states.
Fig: State assignment to modified flow table
ü Multiple-Row Method
The method for making race-free stale assignments by adding extra rows in the flow table is referred to as the shared-row method. A second method called the multiple-row method is not as efficient, but is easier to apply. In multiple- row assignment each state in the original row table is replaced by two or more combinations of state variables.
Fig: Multiple row assignment
There are two binary state variables for each stable state, each variable being the logical complement of the other. For example, the original state a is replaced with two equivalent states a1 =000 and a2 = 111. The output values, not shown here must be the same in a1 and a2. Note that a1 is adjacent to b1, c2 and d1, and a2 is adjacent to c1, b2 and d2, and similarly each state is adjacent to three states with different letter designations.
The expanded table is formed by replacing each row of the original table with two rows. In the multiple-row assignment, the change from one stable state 10 another will always cause a change of only one binary state variable. Each stable stale has two binary assignments with exactly the same output.
A race condition or race hazard is the behavior of an electronics, 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 from the title of a paper presented at the University of Reading to the 2nd National Symposium of Logic Design by D. Zissos on March 28, 1969 under the auspices of The British Computer Society. A race hazard is a circuit transient, which under certain changes of an input signal in a circuit with certain delays, can occur at the output of a gate.
Race conditions can occur especially in logic circuits, and in 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 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.
Critical and non-critical forms
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.
A 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.
Static, dynamic, and essential forms
A static race condition occurs when a signal and its complement are combined together.
A dynamic race condition occurs when it results in multiple transitions when only one is intended. They are due to interaction between gates. It can be eliminated by using no more than two levels of gating.
An essential race condition occurs when an input has two transitions in less than the total feedback propagation time. Sometimes they are cured using inductive delay line elements to effectively increase the time duration of an input signal.
Race conditions arise in software when an application depends on the sequence or timing of processes or threads for it to operate properly. As with electronics, there are critical race conditions that result in invalid execution and bugs. Critical race conditions often happen when the processes or threads depend on some shared state. Operations upon shared states are critical sections that must be mutually exclusive. Failure to obey this rule opens up the possibility of corrupting the shared state.
The memory model defined in the C11 and C++11 standards uses the term "data race" for a race condition caused by potentially concurrent operations on a shared memory location, of which at least one is a write. A C or C++ program containing a data race has undefined behavior.
Race conditions have a reputation of being difficult to reproduce and debug, since the end result is nondeterministic and depends on the relative timing between interfering threads. Problems occurring in production systems can therefore disappear when running in debug mode, when additional logging is added, or when attaching a debugger, often referred to as a "Heisenbug". It is therefore better to avoid race conditions by careful software design rather than attempting to fix them afterwards.
As a simple example, let us assume that two threads want to increment the value of a global integer variable by one. Ideally, the following sequence of operations would take place:
|Thread 1||Thread 2||Integer value|
In the case shown above, the final value is 2, as expected. However, if the two threads run simultaneously without locking or synchronization, the outcome of the operation could be wrong. The alternative sequence of operations below demonstrates this scenario:
|Thread 1||Thread 2||Integer value|
In this case, the final value is 1 instead of the expected result of 2. This occurs because here the increment operations are not mutually exclusive. Mutually exclusive operations are those that cannot be interrupted while accessing some resource such as a memory location.
Many software race conditions have associated computer security implications. A race condition allows an attacker with access to a shared resource to cause other actors that utilize that resource to malfunction, resulting in effects including denial of service and privilege escalation.
A specific kind of race condition involves checking for a predicate (e.g. for authentication), then acting on the predicate, while the state can change between the time of check and the time of use. When this kind of bug exists in security-sensitive code, a security vulnerability called a time-of-check-to-time-of-use (TOCTTOU) bug is created.
Race conditions are also intentionally used to create hardware random number generators and physically unclonable functions. PUFs can be created by designing circuit topologies with identical paths to a node and relying on manufacturing variations to randomly determine which paths will complete first. By measuring each manufactured circuit's specific set of race condition outcomes, a profile can be collected for each circuit and kept secret in order to later verify a circuit's identity.
Two or more programs may collide in their attempts to modify or access a file system, which can result in data corruption or privilege escalation.File locking provides a commonly used solution. A more cumbersome remedy involves organizing the system in such a way that one unique process (running a daemon or the like) has exclusive access to the file, and all other processes that need to access the data in that file do so only via interprocess communication with that one process. This requires synchronization at the process level.
A different form of race condition exists in file systems where unrelated programs may affect each other by suddenly using up available resources such as disk space, memory space, or processor cycles. Software not carefully designed to anticipate and handle this race situation may then become unpredictable. Such a risk may be overlooked for a long time in a system that seems very reliable. But eventually enough data may accumulate or enough other software may be added to critically destabilize many parts of a system. An example of this occurred with the near loss of the Mars Rover "Spirit" not long after landing. A solution is for software to request and reserve all the resources it will need before beginning a task; if this request fails then the task is postponed, avoiding the many points where failure could have occurred. Alternatively, each of those points can be equipped with error handling, or the success of the entire task can be verified afterwards, before continuing. A more common approach is to simply verify that enough system resources are available before starting a task; however, this may not be adequate because in complex systems the actions of other running programs can be unpredictable.
In networking, consider a distributed chat network like IRC, where a user who starts a channel automatically acquires channel-operator privileges. If two users on different servers, on different ends of the same network, try to start the same-named channel at the same time, each user's respective server will grant channel-operator privileges to each user, since neither server will yet have received the other server's signal that it has allocated that channel. (This problem has been largely solved by various IRC server implementations.)
In this case of a race condition, the concept of the "shared resource" covers the state of the network (what channels exist, as well as what users started them and therefore have what privileges), which each server can freely change as long as it signals the other servers on the network about the changes so that they can update their conception of the state of the network. However, the latency across the network makes possible the kind of race condition described. In this case, heading off race conditions by imposing a form of control over access to the shared resource—say, appointing one server to control who holds what privileges—would mean turning the distributed network into a centralized one (at least for that one part of the network operation).
Race conditions can also exist when a computer program is written with non-blocking sockets, in which case the performance of the program can be dependent on the speed of the network link.
Software flaws in life-critical systems can be disastrous. Race conditions were among the flaws in the Therac-25radiation therapy machine, which led to the death of at least three patients and injuries to several more.
Another example is the Energy Management System provided by GE Energy and used by Ohio-based FirstEnergy Corp (among other power facilities). A race condition existed in the alarm subsystem; when three sagging power lines were tripped simultaneously, the condition prevented alerts from being raised to the monitoring technicians, delaying their awareness of the problem. This software flaw eventually led to the North American Blackout of 2003. GE Energy later developed a software patch to correct the previously undiscovered error.
This section needs expansion. You can help by adding to it.(October 2016)
Neuroscience is demonstrating that race conditions can occur in mammal (rat) brains as well.
Many software tools exist to help detect race conditions in software. They can be largely categorized into two groups: static analysis tools and dynamic analysis tools.
Thread Safety Analysis is a static analysis tool for annotation-based intra-procedural static analysis, originally implemented as a branch of gcc, and now reimplemented in Clang, supporting PThreads.[non-primary source needed]
Dynamic analysis tools include: Intel Inspector, a memory and thread checking and debugging tool to increase the reliability, security, and accuracy of C/C++ and Fortran applications; Intel Advisor, a sampling based, SIMD vectorization optimization and shared memory threading assistance tool for C, C++, C#, and Fortran software developers and architects; ThreadSanitizer, which uses binary (Valgrind-based) or source, LLVM-based instrumentation, and supports PThreads);[non-primary source needed] and Helgrind, a Valgrind tool for detecting synchronisation errors in C, C++ and Fortran programs that use the POSIX pthreads threading primitives.[non-primary source needed]
- Karam, G.M.; Buhr, R.J.A. (August 1990). "Starvation and Critical Race Analyzers for Ada". IEEE Transactions on Software Engineering. 16 (8): 829–843. doi:10.1109/32.57622.
- Fuhrer, R.M.; Lin, B.; Nowick, S.M. (March 1995). "Algorithms for the optimal state assignment of asynchronous state machines". Advanced Research in VLSI, 1995. Proceedings., 16th Conference on. pp. 59–75. doi:10.1109/ARVLSI.1995.515611. ISBN 0-8186-7047-9. as PDF
- Paper "A Novel Framework for Solving the State Assignment Problem for Event-Based Specifications" by Luciano Lavagno, Cho W. Moon, Robert K. Brayton, and Alberto Sangiovanni-Vincentelli
- Wheeler, David A. (7 October 2004). "Secure programmer: Prevent race conditions—Resource contention can be used against you"(PDF). IBM developerWorks. Archived from the original(PDF) on Nov 14, 2013.
- Chapter "Avoid Race Conditions" (Secure Programming for Linux and Unix HOWTO)
- Race conditions, security, and immutability in Java, with sample source code and comparison to C code, by Chiral Software
- Karpov, Andrey (11 April 2009). "Interview with Dmitriy Vyukov — the author of Relacy Race Detector (RRD)". Intel Software Library Articles.
- Microsoft Support description
Race condition in a logic circuit. Here, ∆t1 and ∆t2 represent the propagation delays of the logic elements. When the input value A changes from low to high, the circuit outputs a short spike of duration (∆t1 + ∆t2) − ∆t2 = ∆t1.