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Unit 6: Process Synchronization

6.4 Monitors Notes

A monitor is a software synchronization tool with high-level of abstraction that provides a
convenient and effective mechanism for process synchronization. It allows only one process to
be active within the monitor at a time. One simple implementation is shown below.

monitor monitor_name
{
// shared variable declarations
procedure P1 (…) { …. }
…
procedure Pn(…) {……}
Initialization code ( ….) { …}
…
}

6.5 Deadlock

Deadlock occurs when you have a set of processes [not necessarily all the processes in the system],
each holding some resources, each requesting some resources, and none of them is able to obtain
what it needs, i.e. to make progress. Those processes are deadlocked because all the processes
are waiting. None of them will ever cause any of the events that could wake up any of the other
members of the set, and all the processes continue to wait forever. For this model, I assume that
processes have only a single thread and that there are no interrupts possible to wake up a blocked
process. The no-interrupts condition is needed to prevent an otherwise deadlocked process from
being awakened by, say, an alarm, and then causing events that release other processes in the set.
In most cases, the event that each process is waiting for is the release of some resource currently
possessed by another member of the set. In other words, each member of the set of deadlocked
processes is waiting for a resource that is owned by another deadlocked process. None of the
processes can run, none of them can release any resources, and none of them can be awakened.
The number of processes and the number and kind of resources possessed and requested are
unimportant. This result holds for any kind of resource, including both hardware and software.

Figure 6.4: Processes are in Deadlock Situation

Waiting Resource Owned
for X X by B

Process Process
A B

Resource
Owned Y Waiting
by A for Y

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