Transcript slides

Process Synchronization
Deadlock
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
procedure body P1 (…) {
...
}
procedure body P2 (…) {
...
}
procedure body Pn (…) {
...
}
{
initialization code
}
}
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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} state[5];
condition self[5];
void pickup(int i);
void putdown(int i);
void test(int i);
void init() {
for (int i = 0; i < 5; i++)
state[i] = thinking;
}
}
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Dining Philosophers
void pickup(int i) {
state[i] = hungry;
test[i];
if (state[i] != eating)
self[i].wait();
}
void putdown(int i) {
state[i] = thinking;
/* test left and right
neighbors */
test((i+4) % 5);
test((i+1) % 5);
}
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void test(int i) {
if ( (state[(I + 4) % 5] != eating) &&
(state[i] == hungry) &&
(state[(i + 1) % 5] != eating)) {
state[i] = eating;
self[i].signal();
}
}
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Monitor via Semaphores
•
•
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 > 0)
signal(next)
else
signal(mutex);
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For each condition variable x:
semaphore x-sem;
// (initially = 0)
int x-count = 0;
Operation x.wait:
x-count++;
if (next-count > 0)
signal(next);
else
signal(mutex);
wait(x-sem);
x-count--;
Operation x.signal:
if (x-count > 0) {
next-count++;
signal(x-sem);
wait(next);
next-count--;
}
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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 won’t 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, …, Pn–1 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
Rj
•
Pi is holding an instance of 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.
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