What is a race condition

Question

When writing multithreaded applications  one of the most common problems experienced is race conditions     My questions to the community are   What is the race condition  How do you detect them  How do you handle them  Finally  how do you prevent them from occurring

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What is a Race Condition    You are planning to go to a movie at 5 pm  You inquire about the availability of the tickets at 4 pm  The representative says that they are available  You relax and reach the ticket window 5 minutes before the show  I m sure you can guess what happens  it s a full house  The problem here was in the duration between the check and the action  You inquired at 4 and acted at 5  In the meantime  someone else grabbed the tickets  That s a race condition - specifically a  check-then-act  scenario of race conditions      How do you detect them    Religious code review  multi-threaded unit tests  There is no shortcut  There are few Eclipse plugin emerging on this  but nothing stable yet      How do you handle and prevent them    The best thing would be to create side-effect free and stateless functions  use immutables as much as possible  But that is not always possible  So using java util concurrent atomic  concurrent data structures  proper synchronization  and actor based concurrency will help   The best resource for concurrency is JCIP  You can also get some more details on above explanation here

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You can prevent race condition  if you use  Atomic  classes  The reason is just the thread don t separate operation get and set  example is below   AtomicInteger ai   new AtomicInteger 2   ai getAndAdd 5     As a result  you will have 7 in link  ai    Although you did two actions  but the both operation confirm the same thread and no one other thread will interfere to this  that means no race conditions

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There is an important technical difference between race conditions and data races  Most answers seem to make the assumption that these terms are equivalent  but they are not    A data race occurs when 2 instructions access the same memory location  at least one of these accesses is a write and there is no happens before ordering among these accesses  Now what constitutes a happens before ordering is subject to a lot of debate  but in general ulock-lock pairs on the same lock variable and wait-signal pairs on the same condition variable induce a happens-before order    A race condition is a semantic error  It is a flaw that occurs in the timing or the ordering of events that leads to erroneous program behavior    Many race conditions can be  and in fact are  caused by data races  but this is not necessary  As a matter of fact  data races and race conditions are neither the necessary  nor the sufficient condition for one another  This blog post also explains the difference very well  with a simple bank transaction example  Here is another simple example that explains the difference   Now that we nailed down the terminology  let us try to answer the original question   Given that race conditions are semantic bugs  there is no general way of detecting them  This is because there is no way of having an automated oracle that can distinguish correct vs  incorrect program behavior in the general case  Race detection is an undecidable problem   On the other hand  data races have a precise definition that does not necessarily relate to correctness  and therefore one can detect them  There are many flavors of data race detectors  static dynamic data race detection  lockset-based data race detection  happens-before based data race detection  hybrid data race detection   A state of the art dynamic data race detector is ThreadSanitizer which works very well in practice   Handling data races in general requires some programming discipline to induce happens-before edges between accesses to shared data  either during development  or once they are detected using the above mentioned tools   this can be done through locks  condition variables  semaphores  etc  However  one can also employ different programming paradigms like message passing  instead of shared memory  that avoid data races by construction

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Microsoft actually have published a really detailed article on this matter of race conditions and deadlocks  The most summarized abstract from it would be the title paragraph      A race condition occurs when two threads access a shared variable at   the same time  The first thread reads the variable  and the second   thread reads the same value from the variable  Then the first thread   and second thread perform their operations on the value  and they race   to see which thread can write the value last to the shared variable    The value of the thread that writes its value last is preserved    because the thread is writing over the value that the previous thread   wrote

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Consider an operation which has to display the count as soon as the count gets incremented  ie   as soon as CounterThread increments the value DisplayThread needs to display the recently updated value   int i   0    Output    CounterThread - gt  i   1   DisplayThread - gt  i   1   CounterThread - gt  i   2   CounterThread - gt  i   3   CounterThread - gt  i   4   DisplayThread - gt  i   4   Here CounterThread gets the lock frequently and updates the value before DisplayThread displays it  Here exists a Race condition  Race Condition can be solved by using Synchronzation

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A race condition is an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time  but because of the nature of the device or system  the operations must be done in the proper sequence in order to be done correctly   In computer memory or storage  a race condition may occur if commands to read and write a large amount of data are received at almost the same instant  and the machine attempts to overwrite some or all of the old data while that old data is still being read  The result may be one or more of the following  a computer crash  an  illegal operation   notification and shutdown of the program  errors reading the old data  or errors writing the new data

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A sort-of-canonical definition is  when two threads access the same location in memory at the same time  and at least one of the accesses is a write   In the situation the  reader  thread may get the old value or the new value  depending on which thread  wins the race   This is not always a bug mdash in fact  some really hairy low-level algorithms do this on purpose mdash but it should generally be avoided   Steve Gury give s a good example of when it might be a problem

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A race condition is a situation on concurrent programming where two concurrent threads or processes compete for a resource and the resulting final state depends on who gets the resource first

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Race conditions occur in multi-threaded applications or multi-process systems   A race condition  at its most basic  is anything that makes the assumption that two things not in the same thread or process will happen in a particular order  without taking steps to ensure that they do   This happens commonly when two threads are passing messages by setting and checking member variables of a class both can access   There s almost always a race condition when one thread calls sleep to give another thread time to finish a task  unless that sleep is in a loop  with some checking mechanism    Tools for preventing race conditions are dependent on the language and OS  but some comon ones are mutexes  critical sections  and signals   Mutexes are good when you want to make sure you re the only one doing something   Signals are good when you want to make sure someone else has finished doing something   Minimizing shared resources can also help prevent unexpected behaviors  Detecting race conditions can be difficult  but there are a couple signs   Code which relies heavily on sleeps is prone to race conditions  so first check for calls to sleep in the affected code   Adding particularly long sleeps can also be used for debugging to try and force a particular order of events   This can be useful for reproducing the behavior  seeing if you can make it disappear by changing the timing of things  and for testing solutions put in place   The sleeps should be removed after debugging   The signature sign that one has a race condition though  is if there s an issue that only occurs intermittently on some machines   Common bugs would be crashes and deadlocks   With logging  you should be able to find the affected area and work back from there

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A  race condition  exists when multithreaded  or otherwise parallel  code that would access a shared resource could do so in such a way as to cause unexpected results   Take this example   for   int i   0  i  lt  10000000  i          x   x   1       If you had 5 threads executing this code at once  the value of x WOULD NOT end up being 50 000 000   It would in fact vary with each run   This is because  in order for each thread to increment the value of x  they have to do the following   simplified  obviously    Retrieve the value of x Add 1 to this value Store this value to x   Any thread can be at any step in this process at any time  and they can step on each other when a shared resource is involved   The state of x can be changed by another thread during the time between x is being read and when it is written back   Let s say a thread retrieves the value of x  but hasn t stored it yet   Another thread can also retrieve the same value of x  because no thread has changed it yet  and then they would both be storing the same value  x 1  back in x   Example    Thread 1  reads x  value is 7 Thread 1  add 1 to x  value is now 8 Thread 2  reads x  value is 7 Thread 1  stores 8 in x Thread 2  adds 1 to x  value is now 8 Thread 2  stores 8 in x   Race conditions can be avoided by employing some sort of locking mechanism before the code that accesses the shared resource   for   int i   0  i  lt  10000000  i            lock x    x   x   1        unlock x     Here  the answer comes out as 50 000 000 every time   For more on locking  search for  mutex  semaphore  critical section  shared resource

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Here is the classical Bank Account Balance example which will help newbies to understand Threads in Java easily w r t  race conditions   public class BankAccount            param args     int accountNumber  double accountBalance   public synchronized boolean Deposit double amount       double newAccountBalance 0      if amount lt  0           return false            else           newAccountBalance   accountBalance amount          accountBalance newAccountBalance          return true           public synchronized boolean Withdraw double amount       double newAccountBalance 0      if amount gt accountBalance           return false            else          newAccountBalance   accountBalance-amount          accountBalance newAccountBalance          return true           public static void main String   args           TODO Auto-generated method stub     BankAccount b   new BankAccount        b accountBalance 2000      System out println b Withdraw 3000

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Race condition is not only related with software but also related with hardware too  Actually the term was initially coined by the hardware industry  According to wikipedia   The term originates with the idea of two signals racing each other to influence the output first  Race condition in a logic circuit    Software industry took this term without modification  which makes it a little bit difficult to understand  You need to do some replacement to map it to the software world    quot two signals quot    gt   quot two threads quot   quot two processes quot   quot influence the output quot    gt   quot influence some shared state quot   So race condition in software industry means  quot two threads quot   quot two processes quot  racing each other to  quot influence some shared state quot   and the final result of the shared state will depend on some subtle timing difference  which could be caused by some specific thread process launching order  thread process scheduling  etc

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You don t always want to discard a race condition  If you have a flag which can be read and written by multiple threads  and this flag is set to  done  by one thread so that other thread stop processing when flag is set to  done   you don t want that  race condition  to be eliminated  In fact  this one can be referred to as a benign race condition    However  using a tool for detection of race condition  it will be spotted as a harmful race condition   More details on race condition here  http   msdn microsoft com en-us magazine cc546569 aspx

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What is a race condition    The situation when the process is critically dependent on the sequence or timing of other events   For example  Processor A and processor B both needs identical resource for their execution      How do you detect them     There are tools to detect race condition automatically    Lockset-Based Race Checker Happens-Before Race Detection Hybrid Race Detection      How do you handle them     Race condition can be handled by Mutex or Semaphores  They act as a lock allows a process to acquire a resource based on certain requirements to prevent race condition      How do you prevent them from occurring    There are various ways to prevent race condition  such as Critical Section Avoidance    No two processes simultaneously inside their critical regions   Mutual Exclusion  No assumptions are made about speeds or the number of CPUs  No process running outside its critical region which blocks other processes  No process has to wait forever to enter its critical region   A waits for B resources  B waits for C resources  C waits for A resources

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A race condition occurs when two or more threads can access shared data and they try to change it at the same time  Because the thread scheduling algorithm can swap between threads at any time  you don t know the order in which the threads will attempt to access the shared data  Therefore  the result of the change in data is dependent on the thread scheduling algorithm  i e  both threads are  racing  to access change the data    Problems often occur when one thread does a  check-then-act   e g   check  if the value is X  then  act  to do something that depends on the value being X  and another thread does something to the value in between the  check  and the  act   E g   if  x    5     The  Check       y   x   2     The  Act         If another thread changed x in between  if  x    5   and  y   x   2  above        y will not be equal to 10      The point being  y could be 10  or it could be anything  depending on whether another thread changed x in between the check and act  You have no real way of knowing   In order to prevent race conditions from occurring  you would typically put a lock around the shared data to ensure only one thread can access the data at a time  This would mean something like this      Obtain lock for x if  x    5       y   x   2     Now  nothing can change x until the lock is released                    Therefore y   10      release lock for x

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A race condition is a kind of bug  that happens only with certain temporal conditions   Example  Imagine you have two threads  A and B   In Thread A   if  object a    0       object avg   total   object a   In Thread B   object a   0   If thread A is preempted just after having check that object a is not null  B will do a   0  and when thread A will gain the processor  it will do a  divide by zero     This bug only happen when thread A is preempted just after the if statement  it s very rare  but it can happen

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A race condition is an undesirable situation that occurs when two or more process can access and change the shared data at the same time It occurred because  there were conflicting accesses to a resource   Critical section problem may cause race condition  To solve critical condition among the process we have take out only one process at a time which execute the critical section

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Try this basic example for better understanding of race condition       public class ThreadRaceCondition                    param args         throws InterruptedException             public static void main String   args  throws InterruptedException           Account myAccount   new Account 22222222               Expected deposit  250         for  int i   0  i  lt  50  i                  Transaction t   new Transaction myAccount                      Transaction TransactionType DEPOSIT  5 00               t start                          Expected withdrawal  50         for  int i   0  i  lt  50  i                  Transaction t   new Transaction myAccount                      Transaction TransactionType WITHDRAW  1 00               t start                           Temporary sleep to ensure all threads are completed  Don t use in            realworld  -          Thread sleep 1000              Expected account balance is 200         System out println  Final Account Balance                      myAccount getAccountBalance                class Transaction extends Thread        public static enum TransactionType           DEPOSIT 1   WITHDRAW 2            private int value           private TransactionType int value                this value   value                     public int getValue                 return value                        private TransactionType transactionType      private Account account      private double amount                 If transactionType    1  deposit else if transactionType    2 withdraw             public Transaction Account account  TransactionType transactionType              double amount            this transactionType   transactionType          this account   account          this amount   amount             public void run             switch  this transactionType            case DEPOSIT              deposit                printBalance                break          case WITHDRAW              withdraw                printBalance                break          default              System out println  NOT A VALID TRANSACTION                                   public void deposit             this account deposit this amount              public void withdraw             this account withdraw amount              public void printBalance             System out println Thread currentThread   getName                         TransactionType      this transactionType      Amount                      this amount           System out println  Account Balance                      this account getAccountBalance              class Account       private int accountNumber      private double accountBalance       public int getAccountNumber             return accountNumber             public double getAccountBalance             return accountBalance             public Account int accountNumber            this accountNumber   accountNumber                If this method is not synchronized  you will see race condition on        Remove syncronized keyword to see race condition     public synchronized boolean deposit double amount            if  amount  lt  0                return false            else               accountBalance   accountBalance   amount              return true                          If this method is not synchronized  you will see race condition on        Remove syncronized keyword to see race condition     public synchronized boolean withdraw double amount            if  amount  gt  accountBalance                return false            else               accountBalance   accountBalance - amount              return true

[multithreading] What is a race condition?

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