Advanced Synchronization and Deadlock

Size: px

Start display at page:

Download "Advanced Synchronization and Deadlock"

Britton Knight
5 years ago
Views:

1 Advanced Synchronization and Deadlock

2 A house of cards? Locks + CV/signal a great way to regulate access to a single shared object......but general multi-threaded programs touch multiple shared objects How can we atomically modify multiple objects to maintain Safety: prevent applications from seeing inconsistent states Liveness: avoid deadlock a cycle of threads forever stuck waiting for one another

3 Deadlock A cycle of waiting among a set of threads, where each thread is waiting for some other thread in the cycle to take some action Producer() { emptybuffer.acquire() producermutexlock.acquire() : } Producer() { producermutexlock.acquire() emptybuffer.acquire() : }

Deadlock A cycle of waiting among a set of threads, where each thread is waiting for some other thread in the cycle to take some action Mutually recursive locking S waiting for unlock

4 Deadlock A cycle of waiting among a set of threads, where each thread is waiting for some other thread in the cycle to take some action Mutually recursive locking S waiting for unlock lock.acquire() lock.acquire() while (must wait) { waiting for unlock S cv.wait(&lock) } lock.release() lock.release() lock.acquire() lock.acquire() cv.signal() lock.release() lock.release()

5 Deadlock A cycle of waiting among a set of threads, where each thread is waiting for some other thread in the cycle to take some action Mutually recursive locking S waiting for unlock Nested waiting waiting for unlock S S S waiting for signal waiting for unlock

6 Deadlock A cycle of waiting among a set of threads, where each thread is waiting for some other thread in the cycle to take some action Mutually recursive locking S waiting for unlock Nested waiting waiting for unlock S S S waiting for signal waiting for unlock

7 Dining Philosophers N philosophers; N plates; N chopsticks If all philosophers grab right chopstick deadlock!

8 Necessary conditions for deadlock Deadlock only if the all hold Not sufficient in general Bounded resources P A finite number of threads can use a resource; resources are finite No preemption the resource is mine, MINE! (until I release it) waiting for P4 owned by P Wait while holding holds one resource while waiting for another cycle Circular waiting Ti waits for Ti+ and holds a resource requested by Ti- sufficient if one instance of each resource P P

9 Preventing deadlock Remove one of the necessary conditions Provide sufficient resources Removes Bounded resources Preempt resources Removes No preemption Abort requests Removes Wait while holding Atomically acquire all resources Removes Wait while holding Lock ordering Removes Circular waiting

10 Lock ordering A program code convention Developers get together, have lunch, plan lock order Usually reflects static assumptions about the structure of data lock items in a list in order what if order changes? Nothing at compile time or run time prevents violating this convention! Active research on making it better Finding locking bugs Automatically locking things properly Transactional memory

Avoiding Deadlock: The Banker s Algorithm E.W.

loan fulfillments such that all clients receive their

11 Avoiding Deadlock: The Banker s Algorithm E.W. Dijkstra & N. Habermann Sum of maximum resources needs can exceed the total available resources if there exists a schedule of loan fulfillments such that all clients receive their maximal loan build their house pay back all the loan More efficient than acquiring atomically all resources

Living dangerously: Safe, Unsafe, Deadlocked Unsafe Deadlock Safe: For any possible set of resource requests, there exists one safe schedule of processing requests that succeeds in granting all

12 Living dangerously: Safe, Unsafe, Deadlocked Unsafe Deadlock Safe: For any possible set of resource requests, there exists one safe schedule of processing requests that succeeds in granting all pending and future requests no deadlock as long as system can enforce safe schedule Unsafe: There exists a set of (pending and future) resource requests that leads to a deadlock, for any schedule in which requests are processed Safe A system s trajectory through its state space unlucky set of requests can force deadlock Deadlocked: The system has at least one deadlock

13 The Banker s books Max ij = max amount of units of resource R j needed by P i MaxClaim i = Max ij Alloc ij = current allocation of R j held by P i HasNow i = mx j= mx j= Alloc ij Avail j = number of units of R j available A request by P k is safe if there is schedule P, P,...P n such that, for all P i, assuming the request is granted, Xi MaxClaim i -HasNow i Avail + HasNow i j=

14 An Example 5 processes, 4 resources Max Alloc Avail R R R R4 R R R R4 R R R R4 P P P 7 5 P P 5 6 P 5 P4 6 5 P4 6 P P5 4 Is this a safe state? 5

15 An Example 5 processes, 4 resources Max Alloc Avail MaxRequest R R R R4 R R R R4 R R R R4 R R R R4 P P 7 5 P 5 6 P4 6 5 P P P P 5 P P P P4 6 P4 P5 P Is this a safe state? P, P 4, P, P, P 5 While safe sequence does not include all processes: Is there a Pi such that MaxRequesti Avail? if no, exit with unsafe if yes, add Pi to the sequence and set Avail = Avail + HasNowi Exit with safe

16 An Example 5 processes, 4 resources Max Alloc Avail MaxRequest R R R R4 R R R R4 R R R R4 R R R R4 P P P 7 5 P P 5 6 P P4 P5 5 P P P 6 5 P4 6 P P5 4 P P want to change its allocation to Safe? 4

17 An Example 5 processes, 4 resources Max Alloc Avail MaxRequest R R R R4 R R R R4 R R R R4 R R R R4 P P P 7 5 P 4 P 5 6 P P4 P5 P P 5 P 6 5 P4 6 P P5 P P want to change its allocation to Safe?

18 Detecting Deadlock 5 processes, resources. We no longer know Max. Alloc Avail Pending R R R R R R R R R P P P P P P P4 P4 P5 P5 Given the set of pending requests, is there a safe sequence? If no, deadlock

19 Detecting Deadlock 5 processes, resources. We no longer know Max. Alloc Avail Pending R R R R R R R R R P P P P P P P4 P4 P5 P5 Given the set of pending requests, is there a safe sequence? If no, deadlock Can we avoid deadlock by delaying granting requests? Deadlock triggered when request formulated, not granted

A house of cards? Advanced Synchronization and Deadlock. Contra Threads: Events. Contra Threads: Events

A house of cards? Advanced Synchronization and Deadlock Locks + CV/signal a great way to regulate access to a single shared object......but general multi-threaded programs touch multiple shared objects