02/27/2004CSCI 315 Operating Systems Design1 Process Synchronization Deadlock Notice: The slides for this lecture have been largely based on those accompanying the textbook Operating Systems Concepts with Java, by Silberschatz, Galvin, and Gagne (2003). Many, if not all, the illustrations contained in this presentation come from this source.

02/27/2004CSCI 315 Operating Systems Design2 Monitor Definition: High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent processes. A procedure within a monitor can access only local variables defined within the monitor. There cannot be concurrent access to procedures within the monitor (only one thread can be active in the monitor at any given time). Condition variables: queues are associated with variables. Primitives for synchronization are wait and signal. monitor monitor-name { shared variables procedure body P1 (…) {... } procedure body P2 (…) {... } procedure body Pn (…) {... } { initialization code }

02/27/2004CSCI 315 Operating Systems Design3 Monitor To allow a process to wait within the monitor, a condition variable must be declared, as condition x, y; Condition variable can only be used with the operations wait and signal. –The operation x.wait(); means that the process invoking this operation is suspended until another process invokes x.signal(); –The x.signal operation resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect.

02/27/2004CSCI 315 Operating Systems Design4 Monitor and Condition Variables

02/27/2004CSCI 315 Operating Systems Design5 Dining Philosophers with Monitor monitor dp { enum {thinking, hungry, eating} state[5]; condition self[5]; void pickup(int i); void putdown(int i); void test(int i); void init() { for (int i = 0; i < 5; i++) state[i] = thinking; }

02/27/2004CSCI 315 Operating Systems Design6 Dining Philosophers void pickup(int i) { state[i] = hungry; test[i]; if (state[i] != eating) self[i].wait(); } void putdown(int i) { state[i] = thinking; /* test left and right neighbors */ test((i+4) % 5); test((i+1) % 5); } void test(int i) { if ( (state[(I + 4) % 5] != eating) && (state[i] == hungry) && (state[(i + 1) % 5] != eating)) { state[i] = eating; self[i].signal(); }

02/27/2004CSCI 315 Operating Systems Design7 Monitor via Semaphores Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next-count = 0; Each external procedure F will be replaced by wait(mutex); … body of F; … if (next-count > 0) signal(next) else signal(mutex); For each condition variable x: semaphore x-sem; // (initially = 0) int x-count = 0; Operation x.wait: x-count++; if (next-count > 0) signal(next); else signal(mutex); wait(x-sem); x-count--; Operation x.signal: if (x-count > 0) { next-count++; signal(x-sem); wait(next); next-count--; }

02/27/2004CSCI 315 Operating Systems Design8 Concepts to discuss Deadlock Livelock Spinlock vs. Blocking

02/27/2004CSCI 315 Operating Systems Design9 Deadlock: Bridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed up if a deadlock occurs. Starvation is possible.

02/27/2004CSCI 315 Operating Systems Design10 Deadlock: Dining-Philosophers Example Imagine all philosophers start out hungry and that they all pick up their left chopstick at the same time. Assume that when a philosopher manages to get a chopstick, it is not released until a second chopstick is acquired and the philosopher has eaten his share. Question: Why did deadlock happen? Try to enumerate all the conditions that have to be satisfied for deadlock to occur. Question: How could be done to guarantee deadlock won’t happen?

02/27/2004CSCI 315 Operating Systems Design11 A System Model Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices Each resource type R i has W i instances. Each process utilizes a resource as follows: –request –use –release

02/27/2004CSCI 315 Operating Systems Design12 Deadlock Characterization Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. Circular wait: there exists a set {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2, …, P n–1 is waiting for a resource that is held by P n, and P 0 is waiting for a resource that is held by P 0. Deadlock can arise if four conditions hold simultaneously:

02/27/2004CSCI 315 Operating Systems Design13 Resource Allocation Graph The nodes in V can be of two types (partitions): –P = {P 1, P 2, …, P n }, the set consisting of all the processes in the system. –R = {R 1, R 2, …, R m }, the set consisting of all resource types in the system. request edge – directed edge P 1  R j assignment edge – directed edge R j  P i Graph: G=(V,E)

02/27/2004CSCI 315 Operating Systems Design14 Resource Allocation Graph Process Resource Type with 4 instances P i requests instance of R j P i is holding an instance of R j PiPi PiPi RjRj RjRj

02/27/2004CSCI 315 Operating Systems Design15 Example of a Resource Allocation Graph

02/27/2004CSCI 315 Operating Systems Design16 Resource Allocation Graph With A Deadlock

02/27/2004CSCI 315 Operating Systems Design17 Resource Allocation Graph With A Cycle But No Deadlock

02/27/2004CSCI 315 Operating Systems Design18 Basic Facts If graph contains no cycles  no deadlock. If graph contains a cycle  –if only one instance per resource type, then deadlock. –if several instances per resource type, possibility of deadlock.

02/27/2004CSCI 315 Operating Systems Design19 Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and then recover. Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX.

02/27/2004CSCI 315 Operating Systems Design20 Deadlock Prevention Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. –Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. –Low resource utilization; starvation possible. Restrain the ways request can be made.

02/27/2004CSCI 315 Operating Systems Design21 Deadlock Prevention No Preemption – –If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. –Preempted resources are added to the list of resources for which the process is waiting. –Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. Restrain the ways request can be made.