Page 139 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes 4.6.8 Hold and Wait
To ensure that the hold-and-wait condition never occurs in the system, we must guarantee that,
whenever a process requests a resource, it does not hold any other resources. One protocol that
can be used requires each process to request and be allocated all its resources before it begins
execution. We can implement this provision by requiring that system calls requesting resources
for a process precede all other system calls. An alternative protocol allows a process to request
resources only when the process has none. A process may request some resources and use them.
Before it can request any additional resources, however, it must release all the resources that it
is currently allocated. To illustrate the difference between these two protocols, we consider a
process that copies data from a tape drive to a disk file, sorts the disk file, and then prints the
results to a printer. If all resources must be requested at the beginning of the process, then the
process must initially request the tape drive, disk file, and printer. It will hold the printer for its
entire execution, even though it needs the printer only at the end. The second method allows the
process to request initially only the tape drive and disk file. It copies from the tape drive to the
disk, then releases both the tape drive and the disk file. The process must then again request the
disk file and the printer. After copying the disk file to the printer, it releases these two resources
and terminates. These protocols have two main disadvantages. First, resource utilization may
be low, since many of the resources may be allocated but unused for a long period. In the
example given, for instance, we can release the tape drive and disk file, and then again request
the disk file and printer, only if we can be sure that our data will remain on the disk file. If we
cannot be assured that they will, then we must request all resources at the beginning for both
protocols. Second, starvation is possible. A process that needs several popular resources may
have to wait indefinitely, because at least one of the resources that it needs is always allocated
to some other process.

4.6.9 No Preemption
The third necessary condition is that there be no preemption of resources that have already been
allocated. To ensure that this condition does not hold, we can use the following protocol. If a
process is holding some resources and requests another resource that cannot be immediately
allocated to it (that is, the process must wait), then all resources currently being held are
preempted. In other words, these resources are implicitly released. The preempted resources are
added to the list of resources for which the process is waiting. The process will be restarted only
when it can regain its old resources, as well as the new ones that it is requesting. Alternatively,
if a process requests some resources, we first check whether they are available. If they are, we
allocate them. If they are not available, we check whether they are allocated to some other
process that is waiting for additional resources. If so, we preempt the desired resources from
the waiting process and allocate them to the requesting process. If the resources are not either
available or held by a waiting process, the requesting process must wait. While it is waiting,
some of its resources may be preempted, but only if another process requests them. A process
can be restarted only when it is allocated the new resources it is requesting and recovers any
resources that were preempted while it was waiting. This protocol is often applied to resources
whose state can be easily saved and restored later, such as CPU registers and memory space. It
cannot generally be applied to such resources as printers and tape drives.

4.6.10 Circular Wait
The fourth and final condition for deadlocks is the circular-wait condition. One way to ensure
that this condition never holds is to impose a total ordering of all resource types, and to require
that each process requests resources in an increasing order of enumeration.

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