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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 473
442
DEADLOCKS
CHAP. 6
(j)
A
Request R
Request S
Release R
Release S
B
Request S
Request T
Release S
Release T
C
Request T
Request R
Release T
Release R
1. A requests R
2. B requests S
3. C requests T
4. A requests S
5. B requests T
6. C requests R
deadlock
1. A requests R
2. C requests T
3. A requests S
4. C requests R
5. A releases R
6. A releases S
no deadlock
A
R
B
S
C
T
(i)
A
R
B
S
C
T
(h)
A
R
B
S
C
T
(g)
A
R
B
S
C
T
(f)
A
R
B
S
C
T
(e)
(d)
(c)
(b)
(a)
A
R
B
S
C
T
(q)
A
R
B
S
C
T
(p)
A
R
B
S
C
T
(o)
A
R
B
S
C
T
(n)
A
R
B
S
C
T
(m)
A
R
B
S
C
T
(l)
(k)
A
R
B
S
C
T
Figure 6-4.
An example of how deadlock occurs and how it can be avoided.
Page 474
SEC. 6.2
INTRODUCTION TO DEADLOCKS
443
leads to deadlock.
We just carry out the requests and releases step by step, and
after every step we check the graph to see if it contains any cycles. If so, we have a
deadlock; if not, there is no deadlock.
Although our treatment of resource graphs
has been for the case of a single resource of each type, resource graphs can also be
generalized to handle multiple resources of the same type (Holt, 1972).
In general, four strategies are used for dealing with deadlocks.
1.
Just ignore the problem. Maybe if you ignore it, it will ignore you.
2.
Detection and recovery.
Let them occur, detect them, and take action.
3.
Dynamic avoidance by careful resource allocation.
4.
Prevention, by structurally negating one of the four conditions.
In the next four sections, we will examine each of these methods in turn.
6.3 THE OSTRICH ALGORITHM
The simplest approach is the ostrich algorithm: stick your head in the sand and
pretend there is no problem.
†
People react to this strategy in different ways. Math-
ematicians find it unacceptable and say that deadlocks must be prevented at all
costs. Engineers ask how often the problem is expected, how often the system
crashes for other reasons, and how serious a deadlock is.
If deadlocks occur on the
average once every five years, but system crashes due to hardware failures and op-
erating system bugs occur once a week, most engineers would not be willing to
pay a large penalty in performance or convenience to eliminate deadlocks.
To make this contrast more specific, consider an operating system that blocks
the caller when an
open
system call on a physical device such as a Blu-ray driver
or a printer cannot be carried out because the device is busy.
Typically it is up to
the device driver to decide what action to take under such circumstances.
Blocking
or returning an error code are two obvious possibilities.
If one process suc-
cessfully opens the Blu-ray drive and another successfully opens the printer and
then each process tries to open the other one and blocks trying, we have a dead-
lock. Few current systems will detect this.
6.4 DEADLOCK DETECTION AND RECOVERY
A second technique is detection and recovery.
When this technique is used,
the system does not attempt to prevent deadlocks from occurring.
Instead, it lets
them occur, tries to detect when this happens, and then takes some action to
†Actually, this bit of folklore is nonsense. Ostriches can run at 60 km/hour and their kick is powerful
enough to kill any lion with visions of a big chicken dinner, and lions know this.
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