Deadlock - Universitetet i oslo€¦ · Deadlock Concepts Systemdeadlock: nofurtherprogress...

Deadlock

INF2140 Modeling and programming parallel systemsLecture 7

March 06, 2013

INF2140 Modeling and programming parallel systems Lecture 7

Deadlock

Deadlock

Concepts

System deadlock: no further progressFour necessary & sufficient conditions

ModelsDeadlock - no eligible actions

PracticeBlocked threads

Aim: deadlock avoidance - to designsystems where deadlock cannot occur


Deadlock

Deadlock: Four Necessary and Sufficient Conditions

Serially reusable resources:the processes involved share resources which they use undermutual exclusionIncremental acquisition:processes hold on to resources already allocated to them whilewaiting to acquire additional resourcesNo pre-emption:once acquired by a process, resources cannot be pre-empted(forcibly withdrawn) but are only released voluntarilyWait-for cycle:a circular chain (or cycle) of processes exists such that eachprocess holds a resource which its successor in the cycle iswaiting to acquire


Deadlock

Wait-for Cycle

Resources:A,B,C ,D,E

A

B

C D

E

Has A awaits B

Has B awaits C

Has C awaits D Has D awaits E

Has E awaits A


Deadlock

Deadlock Analysis: Primitive Processes

A deadlocked state is one with no outgoing transitionsSTOP process in FSP

MOVE = (north->(south->MOVE|north->STOP)).

Animation to producea trace.

Analysis using LTSA:Shortest trace to STOP

Trace to DEADLOCK:northnorth


Deadlock

Deadlock Analysis: Parallel Composition

In composed systems, deadlock may arise from theparallel composition of interacting processes.

printer: RESOURCE get put

SYS

scanner: RESOURCE get put

p:P printer

scanner

q:Q printer

scanner

Deadlock trace?How to avoid deadlock?

RESOURCE = (get->put->RESOURCE).P = (printer.get->scanner.get

->copy->printer.put->scanner.put->P).

Q = (scanner.get->printer.get->copy->scanner.put->printer.put->Q).

||SYS = (p:P||q:Q||{p,q}::printer:RESOURCE||{p,q}::scanner:RESOURCE).


Deadlock

Deadlock Analysis - Avoidance

1 Acquire resources in the same order?2 Timeout?

P = (printer.get-> GETSCANNER),GETSCANNER = (scanner.get->copy->printer.put->scanner.put->P

|timeout -> printer.put->P).

Q = (scanner.get-> GETPRINTER),GETPRINTER = (printer.get->copy->printer.put->scanner.put->Q

|timeout -> scanner.put->Q).

Deadlock? Progress?


Deadlock

Dining Philosophers

(Dijkstra, 1965)

Five philosophers sit around acircular table.Each philosopher spends his lifealternately thinking and eating.In the centre of the table is alarge bowl of spaghetti.A philosopher needs two forksto eat a helping of spaghetti. 0

1

2 3

4 0

1

2

3

4

One fork is placed between each pair of philosophers and they agreethat each will only use the fork to his immediate right and left.


Deadlock

Dining Philosophers - Model Structure Diagram

Each FORK is a sharedresource with actionsget and put.

When hungry, eachPHIL must first gethis right and left forksbefore he can starteating.


Deadlock

Dining Philosophers - Model

FORK = (get -> put -> FORK).

PHIL = (sitdown ->right.get->left.get->eat ->right.put->left.put->arise->PHIL).

Table of philosophers:

||DINERS(N=5)= forall [i:0..N-1](phil[i]:PHIL ||{phil[i].left,phil[((i-1)+N)%N].right}::FORK).

Can this system deadlock?


Deadlock

Dining Philosophers - Model Analysis

Trace to DEADLOCK:phil.0.sitdownphil.0.right.getphil.1.sitdownphil.1.right.getphil.2.sitdownphil.2.right.getphil.3.sitdownphil.3.right.getphil.4.sitdownphil.4.right.get

This is the situation where all thephilosophers become hungry at thesame time, sit down at the tableand each philosopher picks up thefork to his right.

The system can make no furtherprogress since each philosopher iswaiting for a fork held by hisneighbor, i.e., a wait-for cycle exists!


Deadlock

Dining Philosophers

Deadlock is easily detectedin our model using LTSA

How easy is it to detect apotential deadlock in animplementation?


Deadlock

Dining Philosophers - Implementation in Java

Philosophers:active entities—implementas threadsForks: sharedpassive entities—implementas monitorsDisplay


Deadlock

Dining Philosophers - Fork monitor

taken:encodes thestate of thefork

class Fork {private boolean taken=false;private PhilCanvas display;private int identity;

Fork(PhilCanvas disp , int id){ display = disp; identity = id;}

synchronized void put() {taken=false; display.setFork(identity ,taken);notify (); }

synchronized void get()throws java.lang.InterruptedException {

while (taken) wait ();taken=true; display.setFork(identity ,taken); }

}


Deadlock

Dining Philosophers - Philosopher implementation

class Philosopher extends Thread {... // Follows from the model (we omit sitting downpublic void run() { // and leaving the table)

try {while (true) { // thinking

view.setPhil(identity ,view.THINKING );sleep(controller.sleepTime ()); // hungryview.setPhil(identity ,view.HUNGRY );right.get (); // get right chopstickview.setPhil(identity ,view.GOTRIGHT );sleep (500);left.get(); // eatingview.setPhil(identity ,view.EATING );sleep(controller.eatTime ());right.put (); // return right chopstickleft.put(); // return left chopstick

}} catch (java.lang.InterruptedException e){}}}


Deadlock

Dining Philosophers - implementation in Java

Code to create the philosopherthreads and fork monitors:

for (int i =0; i<N; ++i)fork[i] = new Fork(display ,i);

for (int i =0; i<N; ++i){phil[i] =

new Philosopher(this ,i,fork[(i-1+N)%N],fork[i]);phil[i].start ();

}


Deadlock

Dining Philosophers

To ensure deadlockoccurs eventually, theslider control may bemoved to the left. Thisreduces the time eachphilosopher spendsthinking and eating.

This “speedup” increasesthe probability ofdeadlock occurring.


Deadlock

Deadlock-free Philosophers

Deadlock can be avoided by ensuring that await-for cycle cannot exist. How?

Introduce an asymmetry into ourdefinition of philosophers.

Use the identity i of aphilosopher such that

even numbered philosophersget their left forks firstodd numbered philosophersget their right forks first

Other strategies?

PHIL(I=0)= (when (I%2==0) sitdown

->left.get->right.get->eat->left.put->right.put->arise->PHIL

|when (I%2==1) sitdown->right.get->left.get->eat->left.put->right.put->arise->PHIL).


Deadlock

Maze Example - Shortest Path to “Deadlock”

We can exploit the shortest path trace produced by the deadlockdetection mechanism of LTSA to find the shortest path out of amaze to the STOP process!

We first model the MAZE.

Each position is modelled bythe moves that it permits.The MAZE parameter gives thestarting position.

One example:MAZE(Start=8) = P[Start],P[0] = (north->STOP|east->P[1]),...


Deadlock

Maze Example - Shortest Path to “Deadlock”

||GETOUT = MAZE(7). Shortest path escape tracefrom position 7 ?

Trace to DEADLOCK:eastnorthnorthwestwestnorth


Deadlock

Summary

ConceptsDeadlock: no further progressFour necessary and sufficient conditions:

Serially reusable resourcesIncremental acquisitionNo preemptionWait-for cycle

ModelsNo eligible actions (analysis gives shortest path trace)

PracticeBlocked threads

Aim: deadlock avoidance- to design systems wheredeadlock cannot occur.


Deadlock

Deadlock - Universitetet i oslo€¦ · Deadlock Concepts Systemdeadlock: nofurtherprogress...

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