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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 496
SEC. 6.9
SUMMARY
465
forever. Commonly the event that the processes are waiting for is the release of
some resource held by another member of the set.
Another situation in which
deadlock is possible is when a set of communicating processes are all waiting for a
message and the communication channel is empty and no timeouts are pending.
Resource deadlock can be avoided by keeping track of which states are safe
and which are unsafe.
A safe state is one in which there exists a sequence of
events that guarantee that all processes can finish. An unsafe state has no such
guarantee. The banker’s algorithm avoids deadlock by not granting a request if
that request will put the system in an unsafe state.
Resource deadlock can be structurally prevented by building the system in
such a way that it can never occur by design.
For example, by allowing a process
to hold only one resource at any instant the circular wait condition required for
deadlock is broken. Resource deadlock can also be prevented by numbering all the
resources and making processes request them in strictly increasing order.
Resource deadlock is not the only kind of deadlock.
Communication deadlock
is also a potential problem in some systems although it can often be handled by
setting appropriate timeouts.
Livelock is similar to deadlock in that it can stop all forward progress, but it is
technically different since it involves processes that are not actually blocked. Star-
vation can be avoided by a first-come, first-served allocation policy.
PROBLEMS
1.
Give an example of a deadlock taken from politics.
2.
Students working at individual PCs in a computer laboratory send their files to be
printed by a server that spools the files on its hard disk. Under what conditions may a
deadlock occur if the disk space for the print spool is limited? How may the deadlock
be avoided?
3.
In the preceding question, which resources are preemptable and which are nonpre-
emptable?
4.
In Fig. 6-1 the resources are returned in the reverse order of their acquisition.
Would
giving them back in the other order be just as good?
5.
The four conditions (mutual exclusion, hold and wait, no preemption and circular wait)
are necessary for a resource deadlock to occur. Give an example to show that these
conditions are not sufficient for a resource deadlock to occur. When are these condi-
tions sufficient for a resource deadock to occur?
6.
City streets are vulnerable to a circular blocking condition called gridlock, in which
intersections are blocked by cars that then block cars behind them that then block the
cars that are trying to enter the previous intersection, etc. All intersections around a
city block are filled with vehicles that block the oncoming traffic in a circular manner.
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