11 deadlock

Operating SystemsECE344

Ashvin GoelECE

University of Toronto

Deadlock

2

Resources and Deadlocks

Processes access resources to make progressExamples of computer resourceso

Printerso

Disk driveso

Kernel data structures (e.g., process table, file table)o

Locks and semaphores to protect critical sectionsSuppose there are two processes and two resourceso

Process 1 holds resource A and requests resource Bo

At the same time, Process 2 holds B and requests Ao

Both will become blocked and remain so!o

This is a deadlock

3

Types of Resources

Non-preemptable or Preemptableo

If a non-preemptable

resource is taken away from a process, the process fails

o

A

preemptable

resource can be taken away from a process with no ill effects

Exclusive or Sharedo

Only one process can access an exclusive

resource at a time

o

One or more processes access a shared

resource simultaneously

4

Resource Usage Model

Resource use eventso

Request

the resource (like acquiring a mutex

lock)o

Use

the resourceo

Release

the resource (like releasing a mutex

lock)Response on request denialo

Wait

(either block or busy wait)o

Fail

with error code

5

Deadlock Conditions (informally)

Deadlocks occur when processes are granted exclusive access to non-preemptable resources and wait when a resource request is denied

6

Definition of Deadlock

A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can performUsually the event is the release of a currently held resourceAs a result, none of the processes cano

Be awakenedo

Runo

Release resources

7

Deadlock Conditions

A deadlock situation can occur if and only if the following conditions hold simultaneouslyo

Mutual exclusion condition

–

each resource is assigned to one process

o

Hold and wait condition

–

processes can get more than one resource

o

No preemption condition –

acquired resources cannot be preempted

o

Circular wait condition

–

chain of two or more processes must be waiting for a resource from next process in circular chain

8

Examples of Deadlock

9

Resource Use Examples

Thread_A() {acquire(resource_1);use resource 1;release(resource_1);

}

Mutex mylock1;

lock(mylock1);// Use resource_1unlock(mylock1);

Semaphore sem1 = 1;

wait(sem1);// Use resource_1signal(sem1);

10

One Resource Acquisition Scenario

Thread_A() {acquire(resource_1);use resource 1;release(resource_1);

}

Thread_B() {acquire(resource_2);use resource 2;release(resource_2);

}

No deadlock is possible

11

Two Resource Acquisition Scenario -

1

Thread_A() {acquire(resource_1);acquire(resource_2);use resource 1 and 2;release(resource_2);release(resource_1);

}

Thread_B() {acquire(resource_1);acquire(resource_2);use resource 1 and 2;release(resource_2);release(resource_1);

}


12


2

Thread_A() {acquire(resource_1);use resource 1;release(resource_1);acquire(resource_2);use resource 2;release(resource_2);

}

Thread_B() {acquire(resource_2);use resource 2;release(resource_2);acquire(resource_1);use resource 1;release(resource_1);

}


13


3


}


}

Deadlock is possible!

14

Consequences of Deadlock

Deadlock occurs in a single programo

Kill the programDeadlock occurs in the OSo

User programs can freezeo

System hangs or crasheso

Must restart the system and kill all applications

15

Dealing with Deadlock -

Four Strategies

Ignore the problemo

Advantages, disadvantages?Detection and recoveryAvoidancePrevention

16

Deadlock Detection

Let the problem happen, then detect and recoverHow should a deadlock be detected?Use a resource allocation graph

ResourceR1

P1Process/Thread

“is held by”

“is requesting”

ResourceR2

P2

Deadlock Cycle in the graph

17

Deadlock Detection

Do a depth-first-search for cycles on the resource allocation graph starting at each process node

Deadlock!

18

Deadlock Detection, Multiple Resources

Previous algorithm works with single resource of each type (e.g., printer, scanner)What if there are multiple resources of each type that can be used interchangeably?Need to use a resource allocation matrix

19

Resource Allocation Matrix

Invariant property: Σi=1

Cij

+ Aj

= Ej

20

Detection Algorithm, Multiple Resources

Step 1: Look for some unmarked process Pi, for which the i-th row of R is less or equal to A (using vector comparison)Step 2: If such a process is found, add the i-throw of C to A, mark the process and go back to Step 1If no such process is found, terminate algorithmWhen the algorithm terminates, any unmarkedprocess is deadlocked!

21

Detection Algorithm -

Example

22

Deadlock Detection Issues

How often should the algorithm run?o

On every resource request?o

Periodically?o

When CPU utilization is low?o

When we suspect deadlock because some thread has been asleep for a long period of time?

23

Recovery from Deadlock

If deadlock is detected, what should be done?o

Abort deadlocked processes and reclaim resourceso

Abort one process at a time until deadlock cycle is eliminated

Where should the abort start?o

Lowest priority process?o

Shortest running process?o

Process with fewest resources held?o

Batch processes before interactive processes?o

Minimize number of processes to be terminated?

24

Deadlock Avoidance

Detection vs. Avoidanceo

Detection –

“optimistic” approachAllocate resourcesDetect and fix problem

o

Avoidance –

“pessimistic” approachDon’t allocate resource if it may lead to deadlockIf a process requests a resource make it wait until it is know that there will be no problem

Which one should be used depends upon how easy is it to recover from deadlock?

25

Banker’s Scheduling Algorithm

An extension of the multiple resource detection algorithmUses the notion of a “safe” stateA safe state is one in which there is1.

No deadlock2.

There is some scheduling order by which every process can run to completion even if all of them request their maximum number of units immediately

Banker’s algorithm schedules threads so as to avoid unsafe states => avoids deadlock

26


These are the max. possiblerequests,

which we assumeare known ahead of time!

Invariant property: Σi=1

Cij

+ Aj

= Ej

27


When a resource request comes in, the resource matrices are temporarily updated and then the multiple resource, deadlock detection algorithm is runIf there is no deadlock, the resource is grantedIf there is a deadlock, the resource matrices are rolled back and the request is deferred (the thread needs to wait)

28

Problems with Deadlock Avoidance

The algorithm needs to be run on every resource request!Deadlock avoidance is often impossible because it is hard to know in advance what resources a process will need!Alternative approach is deadlock preventiono

Make deadlock impossible!o

Attack one of the four conditions that are necessary for deadlock to be possible

29

Deadlock Prevention

Conditions necessary for deadlocko

Mutual exclusion condition

–

each resource is assigned to one process

o

Hold and wait condition

–

processes can get more than one resource

o

No preemption condition –

acquired resources cannot be preempted

o

Circular wait condition

–

chain of two or more processes must be waiting for a resource from next process in circular chain

30

Deadlock Prevention

Attacking mutual exclusion?o

A bad idea for most resource types since resource could be corrupted

Attacking hold and wait?o

Require processes to request all resources before they begin!

o

When problems occur a process must release all its resources and start again

Attacking no preemption?o

A bad idea for most resource types since resource could be left in an inconsistent state

31

Deadlock Prevention

Attacking circular waiting?o

Number each of the resourceso

Require each process to acquire lower numbered resources before higher numbered resources

o

More precisely: A process is not allowed to request a resource whose number is lower than the highest numbered resource it currently holds

o

Why does this method work?

32

Recall this Deadlock

Assume that ordering of resources is such that Order(Resource_1) = 1, Order(Resource_2) = 2Thread A violates the ordering!


}


}

33

Resource Ordering

The main problem with ordering resources is that it may be hard to come up with an acceptable ordering of resources!o

Think about third-party softwareStill,this is the most useful approach in an OSExample ordering of resources in Blitz:1.

ProcessControlBlock2.

FileControlBlock3.

Page FramesHowever, the problem of resources with multiple units is not addressed

34

Deadlock, Starvation, Livelock

Deadlocko

A particular set of threads perform no work because of a circular wait condition

o

Once a deadlock occurs, it does not go awayStarvationo

A particular set of threads perform no work because the resources they need are being used by others constantly

o

Starvation can be a temporary conditionLivelocko

A particular set of threads continue to run but make no progress

35

Think Time

What is deadlock?What conditions must hold for deadlock to be possible?What are the main approaches for dealing with deadlock?Why does resource ordering help?

36

Before Next Class

This set of slides cover pages 159-188Reading for next class - pages 189-202

11 deadlock

Documents

mutual exclusion

resource allocation

resource acquisition

wait condition

detection

circular chain

bad idea

safe state