Process Synchronization Deadlock

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Process SynchronizationDeadlock
Notice The slides for this lecture have been
largely based on those accompanying the textbook
Operating Systems Concepts with Java, by
Silberschatz, Galvin, and Gagne (2003). Many, if
not all, the illustrations contained in this
presentation come from this source.
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Monitor

• Definition High-level synchronization construct
  that allows the safe sharing of an abstract data
  type among concurrent processes.

monitor monitor-name shared variables procedur
e body P1 () . . . procedure body P2 ()
. . . procedure body Pn () . .
. initialization code
A procedure within a monitor can access only
local variables defined within the
monitor. There cannot be concurrent access to
procedures within the monitor (only one thread
can be active in the monitor at any given time).
Condition variables queues are associated with
variables. Primitives for synchronization are
wait and signal.
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Monitor

• To allow a process to wait within the monitor, a
  condition variable must be declared, as
• condition x, y
• Condition variable can only be used with the
  operations wait and signal.
• The operation
• x.wait()means that the process invoking this
  operation is suspended until another process
  invokes
• x.signal()
• The x.signal operation resumes exactly one
  suspended process. If no process is suspended,
  then the signal operation has no effect.

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Monitor and Condition Variables
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Dining Philosophers with Monitor

• monitor dp
• enum thinking, hungry, eating state5
• condition self5
• void pickup(int i)
• void putdown(int i)
• void test(int i)
• void init()
• for (int i 0 i lt 5 i)
• statei thinking

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Dining Philosophers

• void pickup(int i)
• statei hungry
• testi
• if (statei ! eating)
• selfi.wait()
• void putdown(int i)
• statei thinking
• / test left and right
• neighbors /
• test((i4) 5)
• test((i1) 5)

void test(int i) if ( (state(I 4) 5 !
eating) (statei hungry)
(state(i 1) 5 ! eating)) statei
eating selfi.signal()
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Monitor via Semaphores
For each condition variable x semaphore
x-sem // (initially 0) int x-count
0 Operation x.wait x-count if
(next-count gt 0) signal(next)
else signal(mutex) wait(x-sem)
x-count-- Operation x.signal if (x-count gt 0)
next-count signal(x-sem) wait(next)
next-count--

• Variables
• semaphore mutex
• // (initially 1)
• semaphore next
• // (initially 0)
• int next-count 0
• Each external procedure F will be replaced by
• wait(mutex)
• body of F
• if (next-count gt 0)
• signal(next)
• else
• signal(mutex)

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Concepts to discuss

• Deadlock
• Livelock
• Spinlock vs. Blocking

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Deadlock Bridge Crossing Example

• Traffic only in one direction.
• Each section of a bridge can be viewed as a
  resource.
• If a deadlock occurs, it can be resolved if one
  car backs up (preempt resources and rollback).
• Several cars may have to be backed up if a
  deadlock occurs.
• Starvation is possible.

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Deadlock Dining-Philosophers Example
Imagine all philosophers start out hungry and
that they all pick up their left chopstick at the
same time. Assume that when a philosopher
manages to get a chopstick, it is not released
until a second chopstick is acquired and the
philosopher has eaten his share. Question Why
did deadlock happen? Try to enumerate all the
conditions that have to be satisfied for deadlock
to occur. Question How could be done to
guarantee deadlock wont happen?
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A System Model

• Resource types R1, R2, . . ., Rm
• CPU cycles, memory space, I/O devices
• Each resource type Ri has Wi instances.
• Each process utilizes a resource as follows
• request
• use
• release

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Deadlock Characterization
Deadlock can arise if four conditions hold
simultaneously

• Mutual exclusion only one process at a time can
  use a resource.
• Hold and wait a process holding at least one
  resource is waiting to acquire additional
  resources held by other processes.
• No preemption a resource can be released only
  voluntarily by the process holding it, after that
  process has completed its task.
• Circular wait there exists a set P0, P1, ,
  P0 of waiting processes such that P0 is waiting
  for a resource that is held by P1, P1 is waiting
  for a resource that is held by
• P2, , Pn1 is waiting for a resource that is
  held by Pn, and P0 is waiting for a resource
  that is held by P0.

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Resource Allocation Graph
Graph G(V,E)

• The nodes in V can be of two types (partitions)
• P P1, P2, , Pn, the set consisting of all
  the processes in the system.
• R R1, R2, , Rm, the set consisting of all
  resource types in the system.
• request edge directed edge P1 ? Rj
• assignment edge directed edge Rj ? Pi

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Resource Allocation Graph

• Process
• Resource Type with 4 instances
• Pi requests instance of Rj
• Pi is holding an instance of Rj

Pi
Rj
Pi
Rj
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Example of a Resource Allocation Graph
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Resource Allocation Graph With A Deadlock
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Resource Allocation Graph With A Cycle But No
Deadlock
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Basic Facts

• If graph contains no cycles ? no deadlock.
• If graph contains a cycle ?
• if only one instance per resource type, then
  deadlock.
• if several instances per resource type,
  possibility of deadlock.

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Methods for Handling Deadlocks

• Ensure that the system will never enter a
  deadlock state.
• Allow the system to enter a deadlock state and
  then recover.
• Ignore the problem and pretend that deadlocks
  never occur in the system used by most operating
  systems, including UNIX.

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Deadlock Prevention
Restrain the ways request can be made.

• Mutual Exclusion not required for sharable
  resources must hold for nonsharable resources.
• Hold and Wait must guarantee that whenever a
  process requests a resource, it does not hold any
  other resources.
• Require process to request and be allocated all
  its resources before it begins execution, or
  allow process to request resources only when the
  process has none.
• Low resource utilization starvation possible.

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Deadlock Prevention
Restrain the ways request can be made.

• No Preemption
• If a process that is holding some resources
  requests another resource that cannot be
  immediately allocated to it, then all resources
  currently being held are released.
• Preempted resources are added to the list of
  resources for which the process is waiting.
• Process will be restarted only when it can regain
  its old resources, as well as the new ones that
  it is requesting.
• Circular Wait impose a total ordering of all
  resource types, and require that each process
  requests resources in an increasing order of
  enumeration.