Deadlocks

Deadlocks

CPE 261403 - Operating Systemshttp://www.e-cpe.org/moodle

Conditions of a Deadlock (7.2.1)

Mutual exclusion

Hold and wait

No preemption

Circular wait

Which of these are in a deadlock?

Methods for Handling Deadlocks Never allow a deadlock state

Allow deadlock and then recover

Ignore the problem

Deadlock Prevention (7.4)

Prevent deadlock conditions from occurring Mutual exclusion

Hold and wait

No preemption

Circular wait

Deadlock Avoidance (7.5)

Figure 7.5

Example

Max Needs Needs at T0(safe)

Needs at T1(unsafe)

P0 4 2 2

P1 10 5 5

P2 9 2 3

Total number of resources = 12

Resource Allocation Graph

Safe Unsafe

Banker’s Algorithm

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

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

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

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

Need [i,j] = Max[i,j] – Allocation [i,j].

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

Example

A B C

7 5 3

3 2 2

9 0 2

2 2 2

4 3 3

P0

P1

P2

P3

P4

A B C

0 1 0

2 0 0

3 0 2

2 1 1

0 0 2

MAX Allocation

3 resource types: A (10 instances), B (5instances), and C (7 instances).

Time T0

Safety Algorithm

Available

Need Finish

P0

P1

P2

P3

P4

R1 R2 R3

Safety Algorithm

7-0=7 5-1=4 3-0=3

3-2=1 2-0=2 2-0=2

9-3=6 0-0=0 2-2=0

2-2=0 2-1=1 2-1=1

4-0=4 3-0=3 3-2=1

3 3 2Available

Need Finish

P0

P1

P2

P3

P4

R1 R2 R3

Example (Cont.)

The content of the matrix Need is defined to be Max – Allocation.

NeedA B C

P0 7 4 3

P1 1 2 2

P2 6 0 0

P3 0 1 1

P4 4 3 1

The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria.

Process P1 makes a request for: 1 instance of A 2 instance of C

Request Vector = (1,0,2)

Safety Algorithm after (1,0,2) request

7 4 3

1 2 2

6 0 0

0 1 1

4 3 1

3 3 2Work

Need Finish

P0

P1

P2

P3

P4

R1 R2 R3

Example: P1 Request (1,0,2)

Check that Request Available (that is, (1,0,2) (3,3,2) true.Allocation Need AvailableA B C A B C A B C

P0 0 1 0 7 4 3 2 3 0

P1 3 0 2 0 2 0

P2 3 0 1 6 0 0

P3 2 1 1 0 1 1

P4 0 0 2 4 3 1 Executing safety algorithm shows that sequence < P1, P3, P4, P0,

P2> satisfies safety requirement. Can request for (3,3,0) by P4 be granted? Can request for (0,2,0) by P0 be granted?

Deadlock Detection (7.6)

Wait-for Graphs (single instance system only)

Resource-Allocation Graph Corresponding wait-for graph

Wait-for Graph does not work with multiple instance resources

Banker’s Algorithm as a deadlock detection tool for multiple instance systems

Deadlock Recovery (7.7)

Process Termination

Abort all deadlock processes

Abort one at a time until deadlock cycle is eliminated

Resource Preemption

Selecting a victim – minimize cost.

Rollback – return to some safe state, restart process for that state.

Starvation – same process may always be picked as victim, include number of rollback in cost factor.