OS Review Chapter 8: Deadlocks

Chapter 7: Deadlocks

A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set.

System resources are utilized in the following way:

• Request: If a process makes a request to use a system resource which cannot be granted immediately, then the requesting process blocks until it can acquire the resource
• Use
• Release

Deadlock Characterization

For a deadlock to occur, each of the following four conditions must hold

• Mutual exclusion: only one process at a time can use a resource.
• Hold and wait: A process must be holding a resource and waiting for another
• No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
• Circular wait: A waits for B, B waits for C, C waits for A.

Resource-Allocation Graph

request edge – directed edge P_i -->R_j

assignment edge – directed edge R_j --> P_i

Resource Allocation Graph With A Deadlock:

Resource Allocation Graph With A Cycle But No Deadlock:

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.

Methods for Handling Deadlocks

• Ensure that the system will never enter a deadlock state

Prevention: Ensure one of the four conditions fails. 確保四個條件之一不滿足

Avoidance: The OS needs more information so that it can determine if the current request can be satisfied or delayed.

• Allow the system to enter a deadlock state, detect it, and recover
• Ignore the problem and pretend that deadlocks never occur in the system 鴕鳥政策，大多數操作系統的處理方式，死鎖的處理十分複雜

Deadlock Prevention: Mutual Exclusion

Some sharable resources must be accessed exclusively which means we cannot deny the mutual exclusion condition. 不容易實現

Deadlock Prevention: Hold and Wait

No process can hold some resources and then request for other resources

• A process must acquire all resources before it runs (hold all and not wait)
• When a process requests for resources, it must hold none (hold none and wait)

–>

• Resource utilization may be low
• Starvation is possible

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

Deadlock Prevention: Circular Wait

example: To break the circular waiting condition, we can order all resource types

A process can only request resources higher than the resource types it holds

A process must release some higher order resources to request a lower order resource

Deadlock Avoidance

Requires that the system has some additional a priori information available

Safe State

system must decide if immediate allocation leaves the system in a safe state

System is in safe state if there exists a safe sequence of all processes

Sequence <P1, P2, …, Pn> is safe if for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j<i.

• If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished.
• When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.
• When Pi terminates, Pi+1 can obtain its needed resources, and so on

Basic Facts

If a system is in safe state --> no deadlocks

If a system is in unsafe state --> possibility of deadlock

Avoidance --> ensure that a system will never enter an unsafe state

Resource-Allocation Graph Algorithm

Claim edge P_i --> R_j indicated that process Pi may request resource R_j; represented by a dashed line(虛線)

Resources must be claimed apriori in the system.資源的需求邊一定要預先提出

Banker’s Algorithm

Let n = number of processes, and m = number of resources types

• Available: Vector of length m. If available [j] = k, there are k instances of resource type R_j available

• Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type R_j.

• Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of R_j

• Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of R_j to complete its task.

Need=Max - Allocation

Safety Algorithm

1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1, …, n.
2. Find an i such that both: (a) Finish [i] = false (b) Needi  Work If no such i exists, go to step 4.
3. Work = Work + Allocationi Finish[i] = true go to step 2.
4. If Finish [i] == true for all i, then the system is in a safe state.

Resource-Request Algorithm for Process Pi

Request = request vector for process P_i. If Request_i[j] = k then process P_i wants k instances of resource type R_j.

1. If Request_i <= Need_i go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim.

2. If Request_i <= Available, go to step 3. Otherwise P_i must wait, since resources are not available

3. Pretend to allocate requested resources to P_i by modifying the state as follows:

   Available = Available - Request_i;

Allocation_i = Allocation_i + Request_i;

Need_i = Need_i – Request_i;

4. If safe -> the resources are allocated to P_i.

   If unsafe -> P_i must wait, and the old resource-allocation state is restored

Detection Algorithm

1. Let Work and Finish be vectors of length m and n, respectively Initialize:

   (a) Work = Available

   (b)For i = 1,2, …, n, if Allocation_i != 0, then Finish[i] = false;otherwise, Finish[i] = true

2. Find an index i such that both:

   (a)Finish[i] == false

   (b)Request_i  Work

   If no such i exists, go to step 4.

3. Work = Work + Allocation_i

   Finish[i] = true

   go to step 2.

4. If Finish[i] == false, for some i, 1 <= i <= n, then the system is in deadlock state. Moreover, if Finish[i] == false, then P_i is deadlocked.

Algorithm requires an order of O(m x n²) operations to detect whether the system is in deadlocked state.

we may not be able to tell which of the many deadlocked processes “caused” the deadlock

Recovery from Deadlock: Process Termination

Recovery from Deadlock: Process Termination

• Priority of the process.
• How long process has computed, and how much longer to completion.
• Resources the process has used.
• Resources process needs to complete.
• How many processes will need to be terminated.
• Is process interactive or batch?

死鎖的檢測和恢復代價都很大，在大多數現有的操作系統中都忽視死鎖