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Modern Operating Systems by Herbert Bos and Andrew...
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Page 491
complete service if their execution were not interleaved with competing processes.
A process locks resources in order to prevent inconsistent resource states caused by
interleaved access to resources. Interleaved access to locked resources, however,
enables resource deadlock.
In Fig. 6-2 we saw a resource deadlock where the re-
sources were semaphores.
A semaphore is a bit more abstract than a Blu-ray drive,
but in this example, each process successfully acquired a resource (one of the
semaphores) and deadlocked trying to acquire another one (the other semaphore).
This situation is a classical resource deadlock.
However, as we mentioned at the start of the chapter, while resource deadlocks
are the most common kind, they are not the only kind.
Another kind of deadlock
can occur in communication systems (e.g., networks), in which two or more proc-
esses communicate by sending messages.
A common arrangement is that process
sends a request message to process
, and then blocks until
sends back a reply
message. Suppose that the request message gets lost.
is blocked waiting for the
is blocked waiting for a request asking it to do something.
We have a
This, though, is not the classical resource deadlock.
does not have posses-
sion of some resource
wants, and vice versa. In fact, there are no resources at all
in sight.
But it is a deadlock according to our formal definition since we have a set
of (two) processes, each blocked waiting for an event only the other one can cause.
This situation is called a
communication deadlock
to contrast it with the more
common resource deadlock. Communication deadlock is an anomaly of
tion synchronization
The processes in this type of deadlock could not complete
service if executed independently.
Communication deadlocks cannot be prevented by ordering the resources
(since there are no resources) or avoided by careful scheduling (since there are no
moments when a request could be postponed).
Fortunately, there is another techni-
que that can usually be employed to break communication deadlocks: timeouts.
most network communication systems, whenever a message is sent to which a re-
ply is expected, a timer is started.
If the timer goes off before the reply arrives, the
sender of the message assumes that the message has been lost and sends it again
(and again and again if needed).
In this way, the deadlock is broken. Phrased dif-
ferently, the timeout serves as a heuristic to detect deadlocks and enables recovery.
This heuristic is applicable to resource deadlock also and is relied upon by users
with temperamental or buggy device drivers that can deadlock and freeze the sys-
Of course, if the original message was not lost but the reply was simply delay-
ed, the intended recipient may get the message two or more times, possibly with
undesirable consequences.
Think about an electronic banking system in which the
message contains instructions to make a payment. Clearly, that should not be re-
peated (and executed) multiple times just because the network is slow or the time-
out too short.
Designing the communication rules, called the
, to get
everything right is a complex subject, but one far beyond the scope of this book.

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