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Processes, Threads, Race Conditions & Deadlocks

Operating Systems Review

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What is a process?

A process is

1. a program in execution.

2. an abstraction of a running program.

3. a data structure that includes everything needed to manage a running program.

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A thread is a lightweight process.

What is a thread?

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Process Hierarchies

A process may create sub-processes, called a “child” process. The process that creates a child process is called the “parent” process.

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Process / Thread States

• Possible process states– running– blocked– ready

• Transitions between states shown

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Implementation of Processes (1)

Fields of a process table entry

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ThreadsThe Thread Model (1)

(a) Three processes each with one thread(b) One process with three threads

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The Thread Model (2)

• Items shared by all threads in a process

• Items private to each thread

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The Thread Model (3)

Each thread has its own stack

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Thread Usage (1)

A word processor with three threads

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Thread Usage (2)

A multithreaded Web server

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Implementing Threads in User Space

A user-level threads package

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Implementing Threads in the Kernel

A threads package managed by the kernel

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Hybrid Implementations

Multiplexing user-level threads onto kernel- level threads

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Interprocess CommunicationRace Conditions

A Race Condition may occur whenTwo or more processes want to access a shared resource at

same time and the result depends upon who runs when.

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Two Atomic Functions (Semaphores)

down(s) up(s)

{ if (s > 0) { if (1 or more processes

s--; waiting for s)

elsepick_one_and_wake_it_up;

go to sleep; else

} s++;

}

These semaphore functions were proposed by E.W. Dijkstra in 1965.

Def’n: An atomic action is an action that is guaranteed to be indivisible – that is, it cannot be interrupted until completed.

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Monitors (1)

Example of a monitor

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Monitors (2)

• Outline of producer-consumer problem with monitors– only one monitor procedure active at one time– buffer has N slots

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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)

Static class our_monitor { // this is a monitorprivate int buffer[] = new int[N];private int count = 0, lo = 0, hi = 0; // counters and indices

public synchronized void insert(int val) {if (count == N) go_to_sleep(); // if the buffer is full, go to sleepbuffer[hi] = val; // insert an item into the bufferhi = (hi + 1) % N; // slot to place next item incount = count + 1; // one more item in the buffer nowif (count == 1) notify(); // if consumer was sleeping, wake it up.

}

public synchronized int remove() {int val;if (count == 0) go_to_sleep(); // if the buffer is empty, go to sleepval = buffer[lo]; // fetch an item from the bufferlo = (lo + 1) % N; // slot to fetch next item fromcount = count – 1; // one less item in the bufferif (count == N-1) notify(); // if producer was sleeping, wake it up.return val;

}

private void go_to_sleep() {try{wait();} catch(InterruptedExecption exc) {};

}}

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Message Passing

The producer-consumer problem with N messages

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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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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

• Usually the event is release of a currently held resource• None of the processes can …

– run

– release resources

– be awakened

Coffman, E.G., Elphick, M.J. and Shoshani, A. 24

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

Holt, R.C.25

Deadlock Modeling (1)

• 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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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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Strategies for dealing with Deadlocks1. just ignore the problem altogether

2. detection and recovery

3. dynamic avoidance • careful resource allocation

4. prevention • negating one of the four necessary conditions

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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 take this approach

• It is a trade off between – convenience– correctness

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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

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Deadlock Prevention

Summary of approaches to deadlock prevention