3.1 KiB
3.1 KiB
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Deadlock
The Deadlock Problem
- A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set.
- Example
- System has 2 disk drives.
- P 1 and P 2 each hold one disk drive and each needs another one.
Bridge Crossing Example
- Traffic only in one direction.
- Each section of a bridge can be viewed as a resource.
- If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback).
- Several cars may have to be backed up if a deadlock occurs.
- Starvation is possible.
Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.
- Mutual exclusion : only one process at a time can use a resource.
- Hold and wait : a process holding at least one resource is waiting to acquire additional resources held by other processes.
- No preemption : a resource can be released only voluntarily by the process holding it, after that process has completed its task.
- Circular wait : there exists a set { P0, P1, …, P0 } of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn –1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.
Methods for Handling Deadlocks
- Put the system in a situation where the deadlock can be avoided, i.e. Deadlock Avoidance.
- Ensure that the system will never enter a deadlock state.
- Allow the system to enter a deadlock state and then recover.
- Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX.
Deadlock Prevention
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Mutual Exclusion – It is not required for sharable resources; must hold for nonsharable resources.
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Hold and Wait – It must guarantee that whenever a process requests a resource, it does not hold any other resources.
- Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none.
- Low resource utilization; starvation possible.
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No Preemption –
- If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released.
- Preempted resources are added to the list of resources for which the process is waiting.
- Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting.
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Circular Wait – It imposes a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.