Day 16
Deadlock
Deadlock



Processes permanently blocked.
Processes are waiting on events, but the
events are to be triggered by blocked
processes.
No simple, efficient solution
Can deadlock occur?
Process P
…
Get A
…
Get B
…
Release A
…
Release B
….
Process Q
…
Get B
…
Get A
…
Release B
…
Release A
….
Joint process diagram (Fig.6.2)
Can deadlock occur?
Process P
…
Get A
…
Release A
…
Get B
…
Release B
….
Process Q
…
Get B
…
Get A
…
Release B
…
Release A
….

The occurrence of deadlock depends on


relative execution rates of processes
details of the application.
Types of resources


Re-usable – returned to the system after use
by a process i.e. not depleted
Examples:
Consumable resources
•
•
Created (produced) and destroyed (consumed) by
processes
Examples:
Example of deadlock with a
consumable resource
Process P
…
Receive (Q)
…
Send (Q, message1)
Process Q
…
Receive (P)
…
Send (P, message2)
• Deadlock occurs if at some point each is waiting to
receive a message from the other, assuming Receive is blocking.
• Caused by a design error.
Pre-emptible resources

Can be taken away from a process without
having to terminate the process and can be
restored to the process at a later time without
altering the behavior of the process.


Examples:
Cannot cause deadlock
Non-pre-emptible resources


If the resource is taken away from the
process, the process will have to be aborted.
The resource must be voluntarily released by
the resource.

Examples:
Resource allocation graphs


A directed graph used to represent the current snapshot of
a system, i.e. current allocations and pending requests.
 A circle represents a process
 A square represents a resource (can have more than one
instance of a resource)
 A graph edge from a resource to a process implies that the
resource is allocated to the process
 A graph edge from a process to a resource implies that the
process is requesting an instance of the resource
Can be used to determine if a system is deadlocked.
Resource allocation graph –
example of a deadlocked system
(Circular wait)
4 conditions for deadlock




Necessary conditions
1.
Mutual exclusion must be enforced.
2.
No preemption – no resource can be forcibly removed from the
process.
3.
Hold and Wait – a process may hold a resource while waiting
for another.
Sufficient condition
4.
Circular wait – closed chain of processes exists, such that each
process holds at least one resource needed by the next
process in the chain.
Conditions 1-3 create a situation that could result in
deadlock
Condition 4 indicates that deadlock has occurred.
Deadlock handling





Prevention
Avoidance
Detection and Recovery
Integrated approach
Ostrich approach
Prevention

Indirect method – prevent any one of the
three (1-3) conditions from existing


Can’t do without enforcing mutual exclusion.
Hold and wait

A process must request all its resources initially. If it
does not get all of them, then release all and block
until all resources simultaneously available.
Prevention

Indirect method

No pre-emption

The process may pre-empt the process that is
holding a resource that the current process wants.


Priority must be used to decide which process should be
pre-empted.
Also, the resource must be a pre-emptible resource.
Prevention


Direct method – prevent circular wait
Consider a linear ordering of resources.



Organize resources into groups that are linearly ordered:
G1 , G2 , G3 … Gn.
A process can only request a resource from G1 and then
from G2 and then from G3 and so on.
No deadlock possible here.