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Implementing Mutual Exclusion

Implementing Mutual Exclusion. Sarah Diesburg Operating Systems COP 4610. From the Previous Lecture. The “too much milk” example shows that writing concurrent programs directly with load and store instructions (i.e., C assignment statements) is tricky

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Implementing Mutual Exclusion

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  1. Implementing Mutual Exclusion Sarah Diesburg Operating Systems COP 4610

  2. From the Previous Lecture • The “too much milk” example shows that writing concurrent programs directly with load and store instructions (i.e., C assignment statements) is tricky • Programmers want to use higher-level operations, such as locks

  3. Ways of Implementing Locks • All implementations require some level of hardware support

  4. Atomic Memory Load and Store • C assignment statements • Examples: “too much milk” solutions

  5. Disable Interrupts (for Uniprocessors) • On a uniprocessor, • An operation is atomic as long as a context switch does not occur in the middle of an operation • Solution 1 Lock::Acquire() { // disable interrupts; } Lock::Release() { // enable interrupts; }

  6. Problems with Solution 1 • A user-level program may not re-enable interrupts • The kernel can no longer regain the control • No guarantees on the duration of interrupts; bad for real-time systems • Solution 1 will not work for more complex scenarios (nested locks)

  7. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } Solution 2

  8. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } The lock is initially FREE. Solution 2

  9. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } Check the lock value while interrupts are disabled. Solution 2

  10. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } Re-enable interrupts inside the loop, so someone may have a chance to unlock. Solution 2

  11. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } Disable the interrupts again before checking the lock. Solution 2

  12. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts while (value != FREE) { // enable interrupts // disable interrupts } value = BUSY; // enable interrupts } Lock::Release() { // disable interrupts value = FREE; // enable interrupts } If no one is holding the lock, grab the lock. Solution 2

  13. Problems with Solution 2 • It works for a single processor • It does not work on a multi-processor machine • Other CPUs can still enter the critical section

  14. The test_and_set Operation • test_and_set works on multiprocessors • Atomically reads a memory location • Sets it to 1 • Returns the old value of memory location

  15. The test_and_set Operation value = 0; Lock::Acquire() { // while the previous value is BUSY, loop while (test_and_set(value) == 1); } Lock::Release() { value = 0; }

  16. Common Problems with Mentioned Approaches • Busy-waiting: consumption of CPU cycles while a thread is waiting for a lock • Very inefficient • Can be avoided with a waiting queue May as well sleep instead of busy-wait…

  17. A tail of two threads… Thread 1: Lazy Thread 2: Lazier Suppose both threads want the lock, but like to be lazy…

  18. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  19. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Thread 1 tries to grab the lock. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  20. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } No more busy waiting… Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  21. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Grab the lock. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  22. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Thread 1 goes on computing. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  23. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Thread 2 tries to grab the lock. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  24. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if(value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } The lock is busy… Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  25. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Put the thread 2 on a waiting queue. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  26. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Sleep it off… Context switch; wait for someone to wake up the thread. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts } zzzz

  27. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Say thread 1 wants to release the lock (interrupts are already disabled by thread 2). Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  28. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Hello? Is someone waiting there? Thread 2 is waiting. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  29. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Put thread 2 on ready queue; context switch. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  30. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Thread 2: Who woke me? I don’t do mornings… Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  31. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Thread 2 is done with its computation. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  32. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Suppose no one else is waiting. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  33. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Release the lock. (Thread 1 has finished its work, so it’s okay.) Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  34. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Warp 9, engage (let’s get out of here)… Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts }

  35. class Lock { int value = FREE; } Lock::Acquire() { // disable interrupts if (value != FREE) { // Queue the thread // Go to sleep } else { value = BUSY; } // enable interrupts } Eventually, the kernel will context switch back to thread 1. Locks Using Interrupt Disables, Without Busy-Waiting Lock::Release() { // disable interrupts if (someone is waiting) { // wake a thread // Put it on ready queue } else { value = FREE; } // enable interrupts } What happened?

  36. So, What’s Going On? • Interrupt disable and enable operations occur across context switches (at the steady state)

  37. Thread A Thread B Context switch Return from sleep Enable interrupts Return from sleep Enable interrupts Context switch So, What’s Going On? Disable interrupts Sleep Disable interrupts Sleep

  38. Locks Using test_and_set, With Minimal Busy-Waiting • Impossible to use test_and_set to avoid busy-waiting • However, waiting can be minimized with a waiting queue

  39. class Lock { int value = FREE; int guard = 0; } Lock::Acquire() { while (test_and_set(guard)); if (value != FREE) { // queue the thread // guard = 0 and sleep } else { value = BUSY; } guard = 0; } Locks Using test_and_set, With Minimal Busy-Waiting Lock::Release() { while (test_and_set(guard)); if (anyone waiting) { // wake up one thread // put it on ready queue } else { value = FREE; } guard = 0; }

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