Name: Deadlocks in Operating Systems
Uploaded: 2026-11-05
Duration: 2 min 19 s
Description: Deadlocks in Operating Systems. Harsh Patle Roll No: 47.

Deadlocks in Operating Systems. Harsh Patle Roll No: 47. 

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What is a Deadlock?. 2. P1. R1. R2. P2. held by P1. 

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A set of processes is deadlocked when each one waits for a resource held by another process in the set. Resource use sequence: Request Use Release No process in the cycle can proceed, so none ever releases its resource.

Necessary Conditions. 3. 1. Mutual exclusion Resource can be used by only one process at a time. 

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Mutual exclusion Resource can be used by only one process at a time

Hold and wait Process holds a resource while waiting for another

No preemption Resource is released only voluntarily by its holder

Circular wait A cycle P0 → P1 → … → Pn → P0 of waiting processes

A deadlock can occur only if all four conditions hold at the same time.

Deadlock Detection. 4. Resource Allocation Graph. 

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Vertices: processes and resources Request edge: P → R Assignment edge: R → P Single instance per resource: a cycle means deadlock Multiple instances: a cycle means possible deadlock

Wait-for graph: remove resource nodes, keep P → P edges Run a cycle-detection algorithm periodically Multi-instance case uses Available, Allocation and Request matrices Frequency depends on how often deadlocks occur and how many processes they affect

Deadlock Avoidance. 5. Safe state A safe sequence of all processes exists. 

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Safe state A safe sequence of all processes exists

Unsafe state No safe sequence; deadlock possible

Processes declare maximum resource needs in advance OS grants a request only if the system stays in a safe state

Matrices: Available, Max, Allocation, Need Safety algorithm finds a safe sequence Resource-request algorithm tests each request

Deadlock Prevention. 6. Ensure at least one of the four necessary conditions can never hold.. 

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Ensure at least one of the four necessary conditions can never hold.

Condition denied How Drawback Mutual exclusion Make resources sharable (e.g. read-only files) Not possible for printers, locks Hold and wait Request all resources at once, or only when holding none Low utilization, starvation No preemption Take resources from a waiting process Works only for savable state (CPU, memory) Circular wait Impose a total ordering; request in increasing order Hard to choose a good order

Recovery & Summary. 7. Recovery from deadlock. Process termination Abort all deadlocked processes Abort one at a time until cycle breaks Resource preemption Select a victim (minimize cost) Rollback to a safe state Prevent starvation of the victim. 

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Process termination Abort all deadlocked processes Abort one at a time until cycle breaks Resource preemption Select a victim (minimize cost) Rollback to a safe state Prevent starvation of the victim

Prevention Break one condition permanently Avoidance Grant requests only in safe states Detection & recovery Allow deadlock, then detect and fix Ignorance Ignore the problem (the "ostrich" approach used by most OSes)

Deadlocks in Operating Systems

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