Atomic Operations and Critical Sections

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Questions and Answers

What is the main purpose of the increment() function as described?

  • To increment a sequence without interruptions. (correct)
  • To allocate memory for a variable.
  • To cause a sequence to reset to zero.
  • To compare two variables for equality.

Why are mutex locks considered a simpler solution compared to previous methods?

  • They allow for unlimited access to the critical section.
  • They require less memory to implement.
  • They provide atomic calls to acquire and release locks. (correct)
  • They operate independently of hardware instructions.

What role does the compare-and-swap instruction play in mutex locks?

  • It ensures locks can be acquired in a non-blocking manner.
  • It facilitates atomic operations for acquiring the lock. (correct)
  • It provides a way to release locks safely.
  • It checks if the lock is not held before allowing access.

In the context of mutex locks, what must be true about the acquire() and release() calls?

<p>They must be atomic in their execution. (C)</p> Signup and view all the answers

How does the use of multiprocessor systems affect mutex lock implementation?

<p>They increase the complexity of lock management. (B)</p> Signup and view all the answers

What is the primary function of a semaphore in process synchronization?

<p>To provide advanced control over process activity synchronization. (C)</p> Signup and view all the answers

What type of variable does a mutex lock typically use to represent its state?

<p>Boolean variable indicating lock availability. (D)</p> Signup and view all the answers

In a continuous loop employing a mutex lock, what should be done immediately after acquiring the lock?

<p>Enter the critical section to perform sensitive operations. (C)</p> Signup and view all the answers

What potential issue can arise if acquire() and release() are not implemented atomically?

<p>Deadlocks or race conditions might occur. (A)</p> Signup and view all the answers

What guarantees does the increment() function provide for the variable it operates on?

<p>The variable will be incremented without interruption or data corruption. (A)</p> Signup and view all the answers

What does the compare_and_swap function do when the current value equals the expected value?

<p>It replaces the current value with the new_value and returns the original value. (A)</p> Signup and view all the answers

Which main property of the compare_and_swap function ensures it works correctly in concurrent environments?

<p>It is executed atomically. (D)</p> Signup and view all the answers

In the provided lock solution using compare_and_swap, what does the condition compare_and_swap(&lock, 0, 1) != 0 indicate?

<p>The lock is currently held by another process. (B)</p> Signup and view all the answers

What type of variables do atomic operations like compare_and_swap typically operate on?

<p>Basic data types such as integers and booleans (C)</p> Signup and view all the answers

What would happen if the compare_and_swap function is called with a non-zero expected value while the current value is zero?

<p>The function returns the current value without any changes. (A)</p> Signup and view all the answers

How does the lock variable get reset in the provided solution using compare_and_swap?

<p>It is explicitly set to 0 after releasing the lock. (D)</p> Signup and view all the answers

What is a key advantage of using compare_and_swap in synchronization algorithms?

<p>It provides a mechanism to manage shared state without deadlocks. (C)</p> Signup and view all the answers

In terms of thread safety, how would you categorize the compare_and_swap instruction?

<p>Thread-safe due to atomic execution. (B)</p> Signup and view all the answers

Which of the following statements about the compare_and_swap function is true?

<p>It is a non-blocking synchronization primitive. (C)</p> Signup and view all the answers

Flashcards

compare_and_swap

A function that atomically compares the value of a variable with a specified expected value and conditionally updates the variable with a new value.

Atomically Executed

The compare_and_swap function is executed as a single, indivisible operation, ensuring that no other thread can interrupt it during its execution.

Return Original Value

The function returns the original value of the variable being compared and potentially updated.

Conditional Update

The variable is updated to the new value only if the expected value matches the current value of the variable.

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Shared Integer Lock

A simple integer variable used as a lock, initialized to 0, to control access to a shared resource.

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Lock Acquisition Loop

A loop that repeatedly uses compare_and_swap to acquire the lock. If the lock is already held, the loop continues to try until it succeeds.

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Critical Section Code

Once the lock is acquired, the critical section code is executed.

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Lock Release

After the critical section is finished, the lock is released, allowing other threads to acquire it.

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Atomic Variable

A variable that allows atomic updates to basic data types like integers and booleans.

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Synchronization Tool

Atomic variables provide a way to ensure that updates to shared data are performed consistently and without interruption, preventing race conditions.

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Mutex Lock

A simple, but powerful synchronization tool used to protect critical sections of code from concurrent access by multiple processes or threads. It acts as a lock that can be acquired by only one process at a time.

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Critical Section

The code segment that requires exclusive access to shared resources and must be executed atomically to prevent data corruption.

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Acquire a Lock

The action of obtaining access to a mutex lock, giving a process exclusive access to the critical section.

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Release a Lock

The action of relinquishing access to a mutex lock after a process has finished executing the critical section.

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Semaphore

A synchronization mechanism that allows processes to coordinate their activities by managing access to shared resources using a counter.

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Remainder Section

The code segment that does not require exclusive access to shared resources and can be executed concurrently with other processes.

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Race Condition

A state where multiple processes can simultaneously access and modify shared resources, potentially leading to data corruption or inconsistent results.

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Event Synchronization

A synchronization technique that allows processes to coordinate their activities by waiting for designated events to occur.

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Wait State

A state where processes are temporarily suspended, waiting for a specific condition or event to occur.

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Running State

A state where processes are actively running and executing their instructions.

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Ready State

A state where processes are loaded into memory but not actively executing their instructions.

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Study Notes

The compare_and_swap Instruction

  • The compare_and_swap instruction is atomic
  • It takes three arguments: a pointer to an integer (value), an expected integer value, and a new_value integer
  • It first reads the current value of the integer pointed to by value into a temporary variable
  • It checks if the current value is equal to the expected value
  • If the values are equal, it updates the integer pointed to by value with the new_value
  • It returns the original value of the integer pointed to by value (the value stored in temp)

Using compare_and_swap to Solve the Critical-Section Problem

  • A shared integer lock, initialized to 0, is used
  • The while loop continuously tries to acquire the lock
  • Inside the loop, compare_and_swap attempts to change the lock value from 0 to 1. If successful, the critical section is entered. If not, the loop continues.
  • Once the critical section is finished, the lock is reset to 0.

Atomic Variables

  • Instructions like compare_and_swap are building blocks for synchronization tools
  • An atomic variable allows uninterruptible updates to data types like integers and booleans
  • An example is sequence, an atomic variable, which can be incremented using the increment() operation

Atomic Variable Implementation

  • The increment() function updates an atomic integer v
  • It enters a do...while loop
  • Inside the loop, it gets the current value of v, stores it in temp
  • It then uses compare_and_swap to attempt to increment v from the value in temp.
  • The loop continues until successful, ensuring atomicity of the increment operation

Mutex Locks

  • Mutex locks are simpler software tools for managing critical sections
  • They use a Boolean variable to indicate whether the lock is available or not
  • Code for critical sections is protected by first acquiring the lock then releasing it
  • Lock acquisition and release operations must be atomic, and are often implemented using hardware instructions like compare-and-swap

Semaphore

  • Semaphores provide more sophisticated synchronization compared to mutexes
  • They are implemented as an integer variable accessible with only two atomic operations: wait() and signal() (also called P() and V()).
  • The wait() operation decrements the integer S; if S is less than or equal to 0, it blocks (busy waits).
  • The signal() operation increments S

Semaphore Types

  • Counting semaphores can have integer values ranging over an unrestricted domain
  • Binary semaphores have integer values between 0 and 1; they behave similarly to mutex locks

Semaphore Usage Example

  • This example shows how semaphores can be used to enforce order of execution between processes
  • By using two semaphores (mutex and synch)

Semaphore Implementation

  • wait and signal operations must guarantee that no two processes can execute them concurrently to avoid race conditions
  • Implementations might use a busy-wait loop within wait, which can be inefficient if critical sections are frequently occupied

Problems with Semaphores

  • Incorrect use of semaphore operations can lead to issues like unintended blocking or missed signals.
  • For example, missing the wait(mutex) or signal(mutex) can produce race conditions
  • Inconsistent use of semaphore operations can lead to critical problems.

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