Necessary Conditions

Figure 7.3: An example to Deadlock.

• A deadlock situation can arise if the following four conditions hold simultaneously in a system:
  • Mutual exclusion. At least one resource must be held in a nonsharable mode;
    • That is, only one process at a time can use the resource.
    • If another process requests that resource, the requesting process must be delayed until the resource has been released.
  • Hold and wait. A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.
  • No preemption. Resources cannot be preempted; that is, a resource can be released only voluntarily by the process holding it, after that process has completed its task.
  • Circular wait. A set $\{P_0,P_1,\ldots,P_n\}$ of waiting processes must exist such that
    • $P_0$ is waiting for a resource held by $P_1$,
    • $P_1$ is waiting for a resource held by $P_2$ ,
    • $\vdots$
    • $P_{n-1}$ is waiting for a resource held by $P_n$,
    • $P_n$ is waiting for a resource held by $P_0$.
    There must be a circular chain of two or more processes, each of which is waiting for a resource held by the next member of the chain.
• We emphasise that all four conditions must hold for a deadlock to occur.