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Announcements

• The fixing the bug part of Lab 4’s assignment 2 is now considered extra credit.

• Comments for the code should be on the parts you wrote for the assignment.

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Concurrency: Deadlock and Starvation

Chapter 6

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Deadlock

• Permanent blocking of a set of processes that either compete for system resources or communicate with each other

• No efficient solution

• Involve conflicting needs for resources by two or more processes

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Deadlock Example

P1 P2 . . Get A Get B

. . Get B Get A . .

Release A Release BRelease B Release A

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Deadlock Example

P1 P2 . .

Get A Get B . .Release A Get A . .Get B Release BRelease B Release A

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Deadlock Environment

• Mutual exclusion– Only one process may use a resource at a

time

• Hold-and-wait– A process may hold allocated resources

while awaiting assignment of others

• No preemption– No resource can be forcibly removed form a

process holding it

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Condition for Deadlock

• Circular wait– A closed chain of processes exists, such that each

process holds at least one resource needed by the next process in the chain

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Deadlock Prevention

• Mutual Exclusion– Must be supported by the operating system

• Hold and Wait– Require a process request all of its required

resources at one time

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Deadlock Prevention

• No Preemption– Process must release resource and request

again– Operating system may preempt a process to

require it releases its resources

• Circular Wait– Define a linear ordering of resource types

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Deadlock Avoidance

• A decision is made dynamically whether the current resource allocation request will, if granted, potentially lead to a deadlock

• Requires knowledge of future process request

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Two Approaches to Deadlock Avoidance

• Do not start a process if its demands might lead to deadlock

• Do not grant an incremental resource request to a process if this allocation might lead to deadlock

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Determination of a Safe StateInitial State

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Determination of a Safe StateP2 Runs to Completion

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Determination of a Safe StateP1 Runs to Completion

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Determination of a Safe StateP3 Runs to Completion

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Resource Allocation Denial

• Referred to as the banker’s algorithm

• State of the system is the current allocation of resources to process

• Safe state is where there is at least one sequence that does not result in deadlock

• Unsafe state is a state that will lead to deadlock if no resources released and rest requested.

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Reusable Resources

• Used by only one process at a time and not depleted by that use

• Processes obtain resources that they later release for reuse by other processes

• Processors, I/O channels, main and secondary memory, devices, and data structures such as files, databases, and semaphores

• Deadlock occurs if each process holds one resource and requests the other

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Consumable Resources

• Created (produced) and destroyed (consumed)

• Interrupts, signals, messages, and information in I/O buffers

• Deadlock may occur if a Receive message is blocking

• May take a rare combination of events to cause deadlock

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Example of Reusable Deadlock

• Deadlock occurs if receive is blocking

P1. . .

. . .Receive(P2);

Send(P2, M1);

P2. . .

. . .Receive(P1);

Send(P1, M2);

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Deadlock AvoidanceSummary

• Maximum resource requirement must be stated in advance

• Processes under consideration must be independent; no synchronization requirements

• There must be a fixed number of resources to allocate

• No process may exit while holding resources

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Sites on Deadlock

• http://access1.sun.com/techarticles/CAT/deadlock.html

• http://www.javaspecialists.co.za/archive/Issue093.html

• http://news.zdnet.com/2100-9595_22-982905.html

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Deadlock Detection

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Strategies once Deadlock Detected

• Abort all deadlocked processes• Back up each deadlocked process to

some previously defined checkpoint, and restart all process– Original deadlock may occur

• Successively abort deadlocked processes until deadlock no longer exists

• Successively preempt resources until deadlock no longer exists

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Selection Criteria Deadlocked Processes

• Least amount of processor time consumed so far

• Least number of lines of output produced so far

• Most estimated time remaining

• Least total resources allocated so far

• Lowest priority

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Strengths and Weaknesses of the Strategies

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Real World Solutions

• Integrated Deadlock Strategy– Group resources into different classes and

enforce a linear ordering strategy.• Prevention (avoiding circular waits)

• Within classes can still have multiple access.

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Example Classes

• Swappable space : Virtual Memory– Must make all requests at the same time.

(Avoiding the Hold-and-Wait problem)

• Process Resources : Assignable devices (ex: tape drives and files)– Know ahead of time (avoidance)– Linear ordering (circular wait nullified)

• Main Memory : – Preemption -> swap out to virtual memory

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Example Classes

• Internal Resources : I/O channels– Resource ordering.

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Dining Philosophers Problem

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Dining Philosophers Problem

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Dining Philosophers Problem

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Dining Philosophers Problem

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Dining Philosophers Problem

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Deadlock in OS

• Most Operating Systems assume deadlock won’t occur.– Within the kernel processes.

• Make all kernel level resource allocations deadlock free.– Get all.– Preemption.– Linear ordering.

• Side Note: Interrupts can play havok in OS design.• How does the kernel determine what is a resource

to be concerned about?

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UNIX Concurrency Mechanisms

• Shared memory– Block of memory that both processes have access

to. Similar to a pipe, with fewer built in controls.

• Pipes– Sending messages but a fixed byte size to store the

message.

• Messages (msgsnd and msgrcv)– Able to store messages in a queue.

– Blocks on send when queue full and on a read when queue empty.

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UNIX Concurrency Mechanisms

• Semaphores (semWait and semSignal)– Can be managed in sets

• semctl : Used to initialize all semaphores in the set to the same value at the same time.

• sem_op : Pass in a set of individual operations to be performed in order.

• Signals– Very similar to an interrupt, but all signals have the

same priority level.– Type determines what happens when received.

• SIGKILL : Kill the process• SIGCHLD : Inform process on the death of a child

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Linux Kernel Concurrency Mechanisms

• Has a set of atomic operations. (atomic_t)– Operations execute without interruption and

without interference.

– atomic_dec_and_test : Subtract a 1 from the variable and return 1 if equals 0.

– atomic_int_and_test : Add a 1 to the variable and return 1 if equals 0.

– atomic_read : Gets the value of the atomic variable.

– atomic_set : Sets the value of the atomic variable to the specified value.

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Linux Kernel Concurrency Mechanisms

• Spinlocks– Very similar to semaphores but designed for shorts

waits.

– Doesn’t put the thread to sleep while waiting for the spinlock to release.

– Can be done with interrupts enabled or disabled.

• Semaphores– Puts the thread to sleep if semaphore unavailable.

– Can be done with interrupts enabled or disable.

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Linux Kernel Concurrency Mechanisms

Used when code is reordered to optimize processor efficiency.

a = 1;b = 1;

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Solaris Thread Synchronization Primitives

• Mutual exclusion (mutex) locks– Basically a binary semaphore.

• Semaphores– Basic counting semaphore.

• Condition variables– Allows a thread to wait until a specific

condition occurs.– Used with a mutex lock.

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Condition Example

mutex_enter(&m)

while (some_condition) {

cv_wait(&cv,&m);

}

mutex_exit(&m)

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Extra Slides

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Example of Deadlock

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Another Example of Deadlock

• Space is available for allocation of 200Kbytes, and the following sequence of events occur

• Deadlock occurs if both processes progress to their second request

P1

. . .

. . .Request 80 Kbytes;

Request 60 Kbytes;

P2

. . .

. . .Request 70 Kbytes;

Request 80 Kbytes;

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Possibility of Deadlock

• Mutual Exclusion

• No preemption

• Hold and wait

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Existence of Deadlock

• Mutual Exclusion

• No preemption

• Hold and wait

• Circular wait

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Determination of an Unsafe State

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Determination of an Unsafe State

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Resource Allocation Graphs

• Directed graph that depicts a state of the system of resources and processes

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Resource Allocation Graphs

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Banker’s Algorithm

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Banker’s Algorithm

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Linux Atomic Operations

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Linux Atomic Operations

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Linux Kernel Concurrency Mechanisms

• Spinlocks– Used for protecting a critical section

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