Deadlock Prevention

Deadlock can be prevented by breaking one of the four necessary conditions:

1. Mutual Exclusion

• Condition: Only one process at a time can hold a resource.
• Prevention: Not needed for sharable resources (like read-only files).
• Required only for non-sharable resources (like printers, files for writing).

2. Hold and Wait

• Condition: A process is holding at least one resource and waiting for others.

• Prevention Strategy:

  • Force processes to request all resources at once
  • OR allow them to request resources only when holding none

  graph TD
    Start[Start]
    RequestAll[Process requests all resources at once]
    Execute[If all available, process executes]
    Wait[If not, process waits and holds nothing]

    Start --> RequestAll --> Execute
    RequestAll --> Wait

3. No Preemption

• Condition: Resources cannot be forcibly taken

• Prevention Strategy:

  • If a process holding resources requests another unavailable resource, all held resources are released
  • The process is suspended until all needed resources (old + new) are available again

  flowchart TD
    A["Process holding R1"]
    B["Requests R2 (unavailable)"]
    C["Release R1"]
    D["Wait for R1 and R2"]
    E["Re-acquire R1 and R2"]
    F["Resume Execution"]

    A --> B --> C --> D --> E --> F

4. Circular Wait

• Condition: A circular chain of waiting processes exists.

• Prevention Strategy:

  • Impose a total ordering on all resources.
  • Processes must request resources in ascending order only.

Example

1Let: R0 < R1 < R2 < R3
2
3Allowed:
4P1 requests R0 → R1 → R3
5
6Disallowed:
7P2 holds R3, requests R0 (violates ascending order)

Visual Representation:

  graph TD
    R0((R0))
    R1((R1))
    R2((R2))
    R3((R3))

    P1((P1)) --> R0
    R0 --> P1
    P1 --> R1
    R1 --> P1
    P1 --> R3
    R3 --> P1

If all processes follow this order, circular wait cannot occur.