1 CSE 513 Introduction to Operating Systems Class 4 - IPC & Synchronization (2) Deadlock Jonathan Walpole Dept. of Comp. Sci. and Eng. Oregon Health and

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CSE 513Introduction to Operating Systems

Class 4 - IPC & Synchronization (2)Deadlock

Jonathan WalpoleDept. of Comp. Sci. and Eng.

Oregon Health and Science University

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Counting semaphores

A binary semaphore can only take on the values of [0, 1].

Class exercise: create a counting semaphore (generalized semaphore that we discussed previously) using just a binary semaphore!!

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Administrivia

Assignments 0 & 1 Next week

midterm exam project review & discussion (Chris Chambers)

Following week Memory Management (Wuchi Feng)

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Possible solution

Semaphore S1, S2, S3; // BINARY!!int C = N; // N is # locks

down_c(sem){ downB(S3); downB(S1); C = C – 1; if (C<0) { upB(S1); downB(S2); } else { upB(S1); } upB(S3); }

up_c(sem){ downB(S1); C = C + 1; if (C<=0) { upB(S2); } upB(S1); }

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Monitors

It is difficult to produce correct programs using semaphores

correct ordering of up and down is tricky! avoiding deadlock is tricky! boundary conditions are tricky!

Can we get the compiler to generate the correct semaphore code for us?

what are suitable higher level abstractions for synchronization?

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Monitors

Collect related shared objects together in a monitor

Characteristics Local data variables are accessible only via the

monitor’s procedures (encapsulation) Processes enter the monitor by invoking one of its

procedures Only one process may execute within the monitor at a

given time (mutual exclusion)

Condition variables (cv) Wait(cv) – process blocked (queued) until condition

holds Signal(cv) – signals the condition and unblocks

(dequeues) a process

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Monitor structures

initializationcode

monitor operations

y

x

shared data

condition queuesmonitor entry queue

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Monitor example for mutual exclusion

process Producerbegin loop <produce char “c”> BoundedBuffer.deposit(c) end loopend Producer

process Consumerbegin loop BoundedBuffer.remove(c) <consume char “c”> end loopend Consumer

monitor: BoundedBuffervar buffer : ...; nextIn, nextOut :... ;

entry deposit(c: char) begin ... end

entry remove(var c: char) begin ... end

end BoundedBuffer

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Monitor example with condition variables

monitor : BoundedBuffervar buffer : array[0..n-1] of char nextIn,nextOut : 0..n-1 := 0 fullCount : 0..n := 0 notEmpty, notFull : condition

entry deposit(c:char) entry remove(var c: char) begin begin if (fullCount = n) then if (fullCount = n) then wait(notFull) wait(notEmpty) end if end if

buffer[nextIn] := c c := buffer[nextOut] nextIn := nextIn+1 mod n nextOut := nextOut+1 mod n fullCount := fullCount+1 fullCount := fullCount-1 signal(notEmpty) signal(notFull) end deposit end removeend BoundedBuffer

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Monitor design choices

Condition variables introduce a problem for mutual exclusion

only one process active in the monitor at a time, so what to do when a process is unblocked on signal?

must not block holding the mutex, so what to do when a process blocks on wait?

Should signals be stored/remembered? signals are not stored if signal occurs before wait, signal is lost!

Should signals count?

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Monitor design choices

Choices when A signals a condition that unblocks B A waits for B to exit the monitor or blocks again B waits for A to exit the monitor or block Signal causes A to immediately exit the monitor or block

(on what condition?)

Choices when A signals a condition that unblocks B & C

B is unblocked, but C remains blocked C is unblocked, but B remains blocked

Choices when A calls wait and blocks a new external process is allowed to enter but which one?

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Common monitor semantics

Hoare semantics On signal, allow signaled process to run; upon

its exit from the monitor, signaler process continues

Brinch Hansen semantics signaler must immediately exit following signal

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Message Passing

Interprocess communication via shared memory across machine boundaries

Message passing can be used locally or remotely for synchronization or general communication

processes use send and receive primitives receive can block like wait send unblocks a process blocked on receive (like

signal unblocking a waiting process)

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Producer consumer with message passing

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Design Choices for Message Passing

Mailboxes system maintains a buffer of sent, but not yet

received, messages Rendezvous

sender and receiver must be active at the same time

receive must be blocked before send occurs kernel does no buffering

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Barriers

Use of a barrier processes approaching a barrier all processes but one blocked at barrier last process arrives, all are let through

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Deadlock

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Resources and Deadlocks

Processes need access to resources in order to make progress

Examples of computer resources printers tape drives kernel data structures (process & file table entries …) locks/semaphores to protect critical sections

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 … this is deadlock

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Resource Usage Model

Sequence of events required to use a resource

request the resource (like acquiring a mutex lock) use the resource release the resource (like releasing a mutex lock)

Must wait if request is denied block busy wait fail with error code

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Preemptable vs Nonpreemptable Resources

Preemptable resources can be taken away from a process with no ill effects

Nonpreemptable resources will cause the process to fail if taken away

Deadlocks occur when processes are granted exclusive access to non-preemptable resources

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Definition of Deadlock

A set of processes is deadlocked if eachprocess in the set is waiting for an event that only

another process in the set can cause

Usually the event is the release of a currently held resource

None of the processes can … be awakened run release resources

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Deadlock conditions

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

Mutual exclusion condition – resource assigned to one process

Hold and wait condition – processes can get more than one resource

No preemption condition

Circular wait condition – chain of two or more processes (must be waiting for resource from next one in chain)

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Resource acquisition scenarios

down (resource_1); use resource_1;up (resource_1);

down (resource_1);down (resource_2); use both resources;up (resource_2);up (resource_1);

down (resource_2); use resource_2;up (resource_2);

down (resource_1);down (resource_2); use both resources;up (resource_2);up (resource_1);

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Resource acquisition scenarios

down (resource_1);down (resource_2); use resources;up (resource_2);up (resource_1);

down (resource_2);down (resource_1); use resources;up (resource_1);up (resource_2);

down (resource_1); use resource;up (resource_1);down (resource_2); use resource;up (resource_2);

down (resource_2); use resource;up (resource_2);down (resource_1); use resource;up (resource_1);

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Flavors of Deadlock

Not so bad Programmer creates a situation that deadlocks Kill the program and move on

Worse Spin locks and locking mechanisms within the

OS

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Other examples of deadlock

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Deadlock modeling

Resource Allocation Graphs (RAGs)

Resource R assigned to process A Process B waiting for resource S Process C and D are deadlocked over T & U

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Dealing with deadlock

Four general strategies Ignore the problem

• Hmm… advantages, disadvantages?

Detection and recovery

Dynamic avoidance through resource allocation

Prevention, by structurally negating one of the four conditions

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Deadlock detection (1 resource of each)

Let the problem happen, then recover

How do you know it happened?

Do a depth-first-search on the resource allocation graph

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Deadlock modeling with multiple resources

Theorem: If a graph does not contain a cycle then no processes are deadlocked

A cycle in a RAG is a necessary condition for deadlock

Is the existence of a cycle a sufficient condition?

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Deadlock modeling with multiple resources

Theorem: If a graph does not contain a cycle then no processes are deadlocked

A cycle in a RAG is a necessary condition for deadlock

Is the existence of a cycle a sufficient condition?

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Deadlock detection (multiple resources)

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Available resource vectorTotal resource vector

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What I am requesting nowWhat I have (now!)

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Detection algorithm

Is there a sequence of running process such that all the resources will be returned?

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Detection algorithm

1. Look for an unmarked process Pi, for which the ith row of R is less than or equal to A

2. If such a process is found, add the i-th row of C to A, mark the process and go back to step 1

3. If no such process exists the algorithm terminates

If all marked, no deadlock

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Detection algorithm

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Detection algorithm

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Detection algorithm

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Detection algorithm

2 2 2 0

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Detection algorithm

2 2 2 0

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Detection algorithm

4 2 2 1 2 2 2 0

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Detection algorithm

4 2 2 1 2 2 2 0

No deadlock!

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Detection algorithms

How often should the algorithm run?

After every resource request?• How many requests types are there?

Periodically?

When CPU utilization is low?

When we suspect deadlock?

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Recovery from deadlock

What should be done to recover? Abort deadlocked processes and reclaim

resources Temporarily reclaim resource, if possible Abort one process at a time until deadlock cycle

is eliminated Where to start?

• Low priority process• How long process has been executing• How many resources a process holds• Batch or interactive• Number of processes that must be

terminated

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Other deadlock recovery techniques

Recovery through rollback

Save state periodically Start computation again from “checkpoint”

Done for large computation systems

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Deadlock avoidance

Detection vs. avoidance…

Detection – “optimistic” approach• Allocate resources• “Break” system to fix it

Avoidance – “pessimistic” approach• Don’t allocate resource if it may lead to

deadlock

Which one to use depends upon the application

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Resource allocation plot

?

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Resource allocation graph

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Safe states

Safe state – “when system 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 resource immediately”

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2

5

10 total

3

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Unsafe states

6

2

5

10 total

3

5

2

5

2

unsafe

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Banker’s algorithm for multiple resources

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

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

Repeat steps 1 and 2, until either all process are marked terminated, in which case the initial state was safe, or until deadlock occurs, in which case it was not

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Avoidance modeling


Maximum Request Vector

Row 2 is what process 2 might need

RUN ALGORITHM ON EVERY RESOURCE REQUEST

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

Max request matrix

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Avoidance algorithm

2 2 2 0

Max request matrix

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Avoidance algorithm

2 2 2 0

Max request matrix

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Avoidance algorithm

4 2 2 1 2 2 2 0

Max request matrix

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Deadlock avoidance

Deadlock avoidance is usually impossible because you don’t know in advance what

resources a process will need!

Alternative approach “deadlock prevention”

Prevent the situation in which deadlock might occur for all time!

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

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Attacking the conditions

Attacking mutual exclusion? a bad idea for some resource types may work for others

Attacking no preemption? a bad idea for some resource types may work for others

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Attacking hold and wait have processes request all resources before

beginning– Underallocation of resources– Unknown requests

if new request, deallocate and reallocate!

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Attacking circular wait same problem/solution in the dining philosophers number all resources and acquire in the same

order may be hard to get an ordering that everyone

likes

1 2 3 4 5 6 7

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Attacking circular wait Same problem in the dining philosophers Number all resources

Typically hard to get an ordering that everyone likes

1 2 3 4 5 6 7

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Attacking circular wait Same problem in the dining philosophers Number all resources

Typically hard to get an ordering that everyone likes

1 2 3 4 5 6 7

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A word on starvation

Starvation and deadlock are two different things

With deadlock – no work is being accomplished for the processes that are deadlocked, because processes are waiting for each other

With starvation – work (progress) is getting done, however, a particular set of processes may not be getting any work done because they cannot obtain the resource they are trying to get

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Summary

What is deadlock?

Deadlock detection algorithms

Read Chapter 3

Sample problems• Chap 3: 15, 20, 21

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Spare slides

Solution to sleeping barber problem.

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Deadlocks

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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Deadlock Modeling (2)

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

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Deadlock Modeling (3)

Strategies for dealing with Deadlocks just ignore the problem altogether detection and recovery dynamic avoidance

• careful resource allocation prevention

• negating one of the four necessary conditions

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A B C

How Deadlock Occurs

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How Deadlock Can be Avoided

(o) (p) (q)

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The Ostrich Algorithm

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

Note the resource ownership and requests A cycle can be found within the graph, denoting

deadlock

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Detection with One Resource of Each Type

Data structures needed by deadlock detection algorithm

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Example Deadlock Detection Algorithm

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Recovery from Deadlock (1)

Recovery through preemption take a resource from some other process 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 (2)

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

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Deadlock AvoidanceResource Trajectories

Two process resource trajectories

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Safe and Unsafe States (1)

Demonstration that the state in (a) is safe

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

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Safe and Unsafe States (2)

Demonstration that the sate in b is not safe

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

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The Banker's Algorithm for a Single Resource

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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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

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 all resources 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 (1)

Normally ordered resources A resource graph

(a) (b)

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Attacking the Circular Wait Condition (1)

Summary of approaches to deadlock prevention

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Other IssuesTwo-Phase Locking

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

program can be stopped, restarted

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Nonresource 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

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Spare Slides

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System calls

Three main parts Providing hooks to “register” the system call Adding the actual system call code Compiling the kernel Making it available to programs

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System calls: header files

/usr/src/linux/include/new_sys_call.h

/usr/src/linux/include/asm-i386/unistd.h

/usr/src/linux/include/arch/i386/kernel/entry.S

#include <linux/linkage.h>#include <linux/unistd.h>_syscall0(int, new_sys_call);

...#define __NR_sys_new_sys_call 243 ...

...

.long SYMBOL_NAME(sys_new_sys_call)

...

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System calls: adding code

/usr/src/linux/kernel/new_sys_call.c

Modify Makefile to update with new files (if any).

Compile and install the kernel

#include <linux/new_sys_call.h>

asmlinkage int sys_new_sys_call(void){ return(222);}

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System calls: creating a program

The most important thing is to have the header file around (as well as the new kernel).

#include <linux/new_sys_call.h>

int main(){

printf(“return val %d\n”,new_sys_call());

}

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Monitors (3)

Solution to producer-consumer problem in Java (part 1)

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Monitors (4)

Solution to producer-consumer problem in Java (part 2)

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Semantics of monitors

What is the strongest statement we can make about the state of the monitor after a waiter wakes up?

entry deposit(c:char) entry remove(var c: char) begin begin if (fullCount = n) then : : wait(notFull) c := buffer[nextOut] fullCount := fullCount-1 end if signal(notFull) : : end deposit end remove

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Synchronization problems with Mesa P1 P2 P3

/* fullCount=n */

if (fullCount==n)

wait(notFull);

remove()

...

fullCount--;

signal(notFull);

...

/* exit monitor */

/* fullCount=n-1*/

deposit()

...

fullCount++;

...

/* exit monitor */

...

fullCount++;

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Mesa semantics

monitor : BoundedBuffervar buffer : array[0..n-1] of char nextIn,nextOut : 0..n-1 := 0 fullCount : 0..n := 0 notEmpty, notFull : condition

entry deposit(c:char) entry remove(var c: char) begin begin while (fullCount = n) then while (fullCount = n) then wait(notFull) wait(notEmpty) end while end while

buffer[nextIn] := c c := buffer[nextOut] nextIn := nextIn+1 mod n nextOut := nextOut+1 mod n fullCount := fullCount+1 fullCount := fullCount-1 signal(notEmpty) signal(notFull) end deposit end removeend BoundedBuffer

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Another deadlock model

Model the state of the computer system as a directed graph G = (V,E)

V = set of vertices = {P1,…,Pn} u {R1,…,Rm}

E = set of edges = {edges from resource to process} u {edges from process to resource}

Pi Ri

Rj

Request edge Allocation edgePi Pk

Documents

1 CSE 513 Introduction to Operating Systems Class 4 - IPC & Synchronization (2) Deadlock Jonathan Walpole Dept. of Comp. Sci. and Eng. Oregon Health and