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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
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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 490
SEC. 6.7
OTHER ISSUES
459
Condition
Approach
Mutual exclusion
Spool everything
Hold and wait
Request all resources initially
No preemption
Take resources away
Circular wait
Order resources numerically
Figure 6-14.
Summary of approaches to deadlock prevention.
6.7.1 Two-Phase Locking
Although both avoidance and prevention are not terribly promising in the gen-
eral case, for specific applications, many excellent special-purpose algorithms are
known. As an example, in many database systems, an operation that occurs fre-
quently is requesting locks on several records and then updating all the locked
records. When multiple processes are running at the same time, there is a real dan-
ger of deadlock.
The approach often used is called
two-phase locking
.
In the first phase, the
process tries to lock all the records it needs, one at a time.
If it succeeds, it begins
the second phase, performing its updates and releasing the locks.
No real work is
done in the first phase.
If during the first phase, some record is needed that is already locked, the proc-
ess just releases all its locks and starts the first phase all over. In a certain sense,
this approach is similar to requesting all the resources needed in advance, or at
least before anything irreversible is done.
In some versions of two-phase locking,
there is no release and restart if a locked record is encountered during the first
phase. In these versions, deadlock can occur.
However, this strategy is not applicable in general.
In real-time systems and
process control systems, for example, it is not acceptable to just terminate a proc-
ess partway through because a resource is not available and start all over again.
Neither is it acceptable to start over if the process has read or written messages to
the network, updated files, or anything else that cannot be safely repeated.
The al-
gorithm works only in those situations where the programmer has very carefully
arranged things so that the program can be stopped at any point during the first
phase and restarted.
Many applications cannot be structured this way.
6.7.2 Communication Deadlocks
All of our work so far has concentrated on resource deadlocks.
One process
wants something that another process has and must wait until the first one gives it
up. Sometimes the resources are hardware or software objects, such as Blu-ray
drives or database records, but sometimes they are more abstract.
Resource dead-
lock is a problem of
competition synchronization
.
Independent processes would
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