1

Deadlocks - Αδιέξοδα

Chapter 3

3.1. Resource 3.2. Introduction to deadlocks 3.3. The ostrich algorithm 3.4. Deadlock detection and recovery 3.5. Deadlock avoidance 3.6. Deadlock prevention 3.7. Other issues

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Resources - Πόροι

• Examples of computer resources– printers

– tape drives

– tables

• Processes need access to resources in reasonable order• Suppose a process holds resource A and requests

resource B– at same time another process holds B and requests A

– both are blocked and remain so

• Hardware and software deadlocks

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Resources

• Deadlocks occur when …

– processes are granted exclusive access to devices– we refer to these devices generally as resources– Resources may have multiple copies

• Preemptable (προεκχωρήσιμοι) resources – can be taken away from a process with no ill effects

(for example memory)

• Nonpreemptable (μη-προεκχωρήσιμοι) resources– will cause the process to fail if taken away (e.g. CDR)

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Resources

• Sequence of events required to use a resource1. request the resource

2. use the resource

3. release the resource

• Must wait if request is denied– requesting process may be blocked– may fail with error code

• Nature of requesting a resource is highly system dependent (e.g. request system call)

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Resource Acquisitiont

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Introduction to Deadlocks

• Formal definition :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 cause

• Only one thread, no interrupts• Usually the event is release of a currently held resource• None of the processes can …

– run– release resources– be awakened

• Number of processes and resources is unimportant

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Four Conditions for Deadlock

1. Mutual exclusion condition• each resource assigned to 1 process or is available

2. Hold and wait condition• process holding resources can request additional

3. No preemption condition• previously granted resources cannot forcibly taken away

4. Circular wait condition• must be a circular chain of 2 or more processes• each is waiting for resource held by next member of the

chain

• All relate to a policy that a system can or can not have

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Deadlock Modeling

• Modeled with directed graphs

– resource R assigned to process A– process B is requesting/waiting for resource S– process C and D are in deadlock over resources T and U

9How deadlock occurs

A B CDeadlock Modeling

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Deadlock Modeling

How deadlock can be avoided

(o) (p) (q)

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Deadlock Modeling

Strategies for dealing with Deadlocks1. just ignore the problem altogether

2. detection and recovery (ανίχνευση και επανόρθωση)

3. dynamic avoidance (αποφυγή)• careful resource allocation

4. prevention (πρόληψη)• negating one of the four necessary conditions

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The Ostrich Algorithm – Αλγ. στρουθοκαμήλου

• Pretend there is no problem

• Reasonable if – deadlocks occur very rarely – cost of prevention is high

• UNIX and Windows takes this approach

• It is a trade off between – convenience– correctness

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Detection with One Resource of Each Type

• A…G: processes; R…W: resources• Note the resource ownership and requests• Is this system deadlocked and if yes, which processes are involved?• A cycle can be found within the graph, denoting deadlock

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Detection with One Resource of Each Type

• We need a formal algorithm for detecting deadlocks• A simple one to detect cycles:

– Take each node in turn.

– Do a DFS (depth first search) on it.

– If it comes to a node it has encountered in this run, then there exists a cycle.

• Previous graph has a cycle

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Detection with Multiple Resources of Each Type

Data structures needed by deadlock detection algorithm

At all times: Σi=1Cij + Aj = Ej

n

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Detection with Multiple Resources of Each Type

• Deadlock detection is based on comparing vectors:

• Algorithm– Look for an unmarked process, Pi for which the i-th

row of R is less or equal to A– If such a process is found, add the i-th row of C to

A, mark the process and go back to step 1– If no such process exists the algorithm terminates

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Detection with Multiple Resources of Each Type

An example for the deadlock detection algorithm(3/2/1)

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Detection with Multiple Resources of Each Type

• When to look for deadlocks?– Every time a resource request is made

• Detection ASAP

• Expensive

– Every k minutes or whenever the CPU utilization drops below a certain threshold

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Recovery from Deadlock - Επανόρθωση

• Recovery through preemption– take a resource from some other process (e.g. printer)– depends on nature of the resource

• Recovery through rollback– checkpoint a process periodically– use this saved state – restart the process if it is found deadlocked

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Recovery from Deadlock

• Recovery through killing processes– crudest but simplest way to break a deadlock– kill one of the processes in the deadlock cycle– the other processes get its resources – choose process that can be rerun from the beginning

(perhaps not in cycle)

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Deadlock Avoidance - Αποφυγή

• So far we assumed that all requests take place at the beginning

• The system must be able to decide whether granting a resource request is safe or not

• Is there an algorithm that can always avoid deadlocks?

• Yes, if certain information is known in advance

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Deadlock Avoidance - Resource Trajectories

• Two process resource trajectories• /// and \\\ are impossible to get• What scheduler should do at point t ?

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Safe and Unsafe States

Demonstration that the state in (a) is safe – 10 instances

(a) (b) (c) (d) (e)

• A state is said to be safe if it is not deadlocked and there is some scheduling order in which every process can run to completion even if all of them suddenly request their maximum number of resources immediately

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Safe and Unsafe States

• Demonstration that the state in b is not safe

• An unsafe state is not a deadlocked state

(a) (b) (c) (d)

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The Banker's Algorithm for a Single Resource

• Check to see if granting the request leads to unsafe state• Three resource allocation states

– safe– safe– unsafe

(a) (b) (c)

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Banker's Algorithm for Multiple Resources

Example of banker's algorithm with multiple resources

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Banker's Algorithm for Multiple Resources

1. Look for a row, R, whose unmet resource needs are all smaller than or equal to A. If no such row exists the system will eventually deadlock since no process can run to completion.

2. Assume the process of the row chosen requests all the resources it needs (which is guaranteed to be possible) and finishes. Mark that process as terminated and add all its resources to the A vector

3. Repeat step 1 and 2 until either all processes are marked terminated, in which case the state is safe, or until a deadlock occurs, in which case is not.

• B requests a printer… (D,A or E, …)• E requests a printer… (deadlock)

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Deadlock PreventionAttacking the Mutual Exclusion Condition

• Some devices (such as printer) can be spooled– only the printer daemon uses printer resource– thus deadlock for printer eliminated

• Not all devices can be spooled• Principle:

– avoid assigning resource when not absolutely necessary

– as few processes as possible actually claim the resource

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Attacking the Hold and Wait Condition

• Goal: Prevent processes that hold resources from waiting for more resources

• Require processes to request resources before starting– a process never has to wait for what it needs

• Problems– may not know required resources at start of run– also ties up resources other processes could be using

• Variation: – process must give up temporarily all resources before

requesting a new one– then request all immediately needed

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Attacking the No Preemption Condition

• This is not a viable option

• Consider a process given the printer– halfway through its job– now forcibly take away printer– !!??

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Attacking the Circular Wait Condition

• Normally ordered resources• A resource graph

(a) (b)

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Attacking the Circular Wait Condition

• Rule: All requests of a process must be made in numerical order => the resource allocation graph can not have cycles

• Either i < j or i > j => can’t have deadlocks• Same logic with multiple resources: at every

instant one assigned resource will be the highest

• Problem: impossible to find an ordering to satisfy everyone

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Attacking the Circular Wait Condition

Summary of approaches to deadlock prevention

• Avoidance and prevention are not widely used in OS, but have special-purpose applications

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Other IssuesTwo-Phase Locking

• DB systems lock records for update• Phase One

– process tries to lock all records it needs, one at a time– if needed record found locked, start over– (no real work done in phase one)

• If phase one succeeds, it starts second phase, – performing updates– releasing locks

• Note similarity to requesting all resources at once• Algorithm works where programmer can arrange

things so that the program can be stopped and restarted

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Non-resource Deadlocks

• Possible for two processes to deadlock– each is waiting for the other to do some task

• Can happen with semaphores– each process required to do a down() on two

semaphores (mutex and another)– if done in wrong order, deadlock results

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Starvation

• Algorithm to allocate a resource – may be to give to shortest job first

• Works great for multiple short jobs in a system

• May cause long job to be postponed indefinitely– even though not blocked

• Solution:– First-come, first-serve policy