Race Conditions

1
Race Conditions

• When threads share access to a common
  object/data, they can conflict with each other
  and mess up the consistency of the object/data.
• ThreadUnsafeBankDeposit
• Desirable output
• Current balance (d) 0.0, Depositing 100.0, New
  balance (d) 100.0
• Current balance (w) 100.0, Withdrawing 100.0,
  New balance (w) 0.0
• Current balance (d) 0.0, Depositing 100.0, New
  balance (d) 100.0
• Current balance (w) 100.0, Withdrawing 100.0,
  New balance (w) 0.0
• Current balance (d) 0.0, Depositing 100.0, New
  balance (d) 100.0
• Current balance (d) 100.0, Depositing 100.0, New
  balance (d) 200.0
• Current balance (w) 200.0, Withdrawing 100.0,
  New balance (w) 100.0
• Reality
• Current balance (w) 0.0Current balance (d) 0.0,
  New balance (d) 100.0
• , New balance (w) -100.0

2

• Current balance (w) 0.0Current balance (d) 0.0,
  New balance (d) 100.0
• , New balance (w) -100.0

Deposit thread
Withdraw thread
print(current balance) // balance0 newBalance
balance - 100
Reaches the end ofits time slice
(newBalance-100, balance0)
Gain a time slice
print(current balance)// balance0newBalance
balance100println(new balance) //
newBalance100 balance newBalance // balance100
Gain a time slice
println(new balance) // newBalance-100 balanc
e newBalance // balance-100
3

• Current balance (w) 0.0Current balance (d) 0.0,
  New balance (d) 100.0
• , New balance (w) -100.0

Deposit thread
Withdraw thread
print(current balance) // balance0 newBalance
balance - 100
Reaches the end ofits time slice
(newBalance-100, balance0)
Gain a time slice
print(current balance)// balance0newBalance
balance100println(new balance) //
newBalance100 balance newBalance // balance100
Gain a time slice
println(new balance) // newBalance-100 balanc
e newBalance // balance-100
newBalance and balance should have been
synchronized.
4

• ThreadUnsafeBankAccount2
• Removed a local variable (newBalance) from
  ThreadUnsafeBankAccount
• to remove a risk to fail data synchronization
  between balance and newBalance
• Output
• Current balance (d) 0.0, New balance (d) 100.0
• Current balance (w) 100.0, New balance (w) 0.0
• Current balance (d) 0.0, New balance (d) 100.0
• Current balance (w) 100.0, New balance (w) 0.0
• Current balance (d) 0.0Current balance (w) 0.0,
  New balance (w) -100.0
• , New balance (d) 100.0

5

• Current balance (d) 0.0Current balance (w) 0.0,
  New balance (w) -100.0
• , New balance (d) 100.0

Withdraw thread
Deposit thread
print(current balance) // 0 balance100 // --gt
100
Reaches the end ofits time slice (balance0)
Gain a time slice
print(current balance) // 0balance balance
- amount// balance-100println(new balance)
// -100
Gain a time slice
Reaches the end ofits time slice (balance-100)
balance 100 println(new balance)//
balance100
6

• Current balance (d) 0.0Current balance (w) 0.0,
  New balance (w) -100.0
• , New balance (d) 100.0

Withdraw thread
Deposit thread
print(current balance) // 0 balance100 // --gt
100
Reaches the end ofits time slice (balance0)
Gain a time slice
print(current balance) // 0balance balance
- amount// balance-100println(new balance)
// -100
Gain a time slice
Reaches the end ofits time slice (balance-100)
balance 100 println(new balance)//
balance100
Balance should have been synchronized between
threads.
7

• All threads
• Run in their race to complete their tasks.
• Manipulate a shared object/data.
• The end result depends on which of them happens
  to win the race.
• No guarantees on the order of thread execution.
• No guarantees on the end result on shared data.

8
Thread Synchronization

• Synchronizing threads, or synchronizing
  object/data access
• Serialized/atomic access to a shared object/data
• Atom the smallest possible unit of matter
  unable to be broken into separate parts
• Atomic code cannot be interrupted during its
  execution. Any intermediate result/state cannot
  be revealed to other entities.
• Atomic a 0 a a 3

9
Lock

• Lock
• Used to control the threads that want to
  manipulate a shared resource data/object.
• java.util.concurrent.locks.Lock interface
• ReentrantLock class most commonly-used lock
  class
• A lock is added to a class whose methods access a
  shared data/object.
• Atomic code is surrounded by lock() and unlock()
  method calls.
• aLock new ReentrantLock()aLock.lock()atomic
  code aLock.unlock()

10

• When a thread calls lock(),
• it owns the lock until it calls unlock().
• No other threads cannot execute atomic code.
• It is guaranteed to execute all atomic code
  before calling unlock().
• If a thread calls lock() when another thread owns
  the lock,
• it goes to the blocked state.

11
Waiting
TimedWaiting
sleep()join()wait()await()
notify()notifyAll()signalAll()interruption
notify()notifyAll()signalAll() interruption
sleep()join()wait()await()
Exits run() or Explicit threadtermination
Runnable
Terminated
new
New
start()
I/O op completionor thread sync done
I/O operation or wait for thread sync (lock)
Blocked
12

• The thread scheduler
• Periodically reactivates each blocked thread so
  that it can acquire a target lock.
• If the lock is still unavailable, the thread is
  again blocked.
• Notifies the completion of atomic code to waiting
  threads.
• Choose one of the waiting thread to acquire the
  lock.
• Eventually, when the target lock is available,
  the waiting thread can acquire the lock.

13
A Locking Idiom

• Call unlock() in a finally clause.
• aLock new ReentrantLock()aLock.lock()try
  atomic code finally aLock.unlock()
• If atomic code throws an exception, unlock() is
  never called.
• A deadlock occurs.
• Atomic code is locked forever, and nobody can
  acquire the lock to execute the atomic code.

14
ThreadSafeBankAccount

• Output
• Lock obtained
• Current balance (d) 0.0, New balance (d) 100.0
• Lock released
• Lock obtained
• Current balance (w) 100.0, New balance (w) 0.0
• Lock released
• Lock obtained
• Current balance (d) 0.0, New balance (d) 100.0
• Lock released
• Lock obtained
• Current balance (w) 100.0, New balance (w) 0.0
• Lock released

15
Nested Locking

• class BankAccount private double
  balance private ReentrantLock lock public
  void deposit(double amount) lock.lock() bal
  ance amount if (balance lt
  MIN_BALANCE) subractPenaltyFee() lock.unlock
  () private void subractPenaltyFee()
  balance - PENALTY

16

• class BankAccount private double
  balance private ReentrantLock lock public
  void deposit(double amount) lock.lock() bala
  nce amount notifyCustomer(this) lock.unlo
  ck() public void notifyCustomer(BankAccount
  account) customer.notify(account) public
  double getBalance() return balance
• class Customer public void notify(BankAccount
  account) System.out.print( account.getBalance()
  )

17

• class BankAccount private double
  balance private ReentrantLock lock public
  void deposit(double amount) lock.lock() bal
  ance amount if (balance lt
  MIN_BALANCE) subractPenaltyFee() lock.unlock
  () public void subractPenaltyFee()
  lock.lock() balance - PENALTY
  lock.unlock()

18

• class BankAccount private double
  balance private ReentrantLock lock public
  void deposit(double amount) System.out.println
  (lock.getHoldCount()) lock.lock() balance
  amount System.out.println(lock.getHoldCount(
  )) if (balance lt MIN_BALANCE) subractPenalty
  Fee() lock.unlock() System.out.println(loc
  k.getHoldCount()) public void
  subractPenaltyFee() lock.lock() System.out.
  println(lock.getHoldCount())
• balance - PENALTY lock.unlock()
  System.out.println(lock.getHoldCount())

19

• class BankAccount private double
  balance private ReentrantLock lock public
  deposit(double amount) lock.lock() try
• balance amount if (balance lt
  MIN_BALANCE) subractPenaltyFee() finally
  lock.unlock() assert lock.getHoldCount()
  0 ERROR public subractPenaltyFee()
  balance - PENALTY