Deadlock

In concurrent computing, a deadlock is a state in which each member of a group of actions, is waiting for some other member to release a lock. Deadlock is a common problem in multiprocessing systems, parallel computing, and distributed systems, where software and hardware locks are used to handle shared resources and implement process synchronization.

In an operating system, a deadlock occurs when a process or thread enters a waiting state because a requested system resource is held by another waiting process, which in turn is waiting for another resource held by another waiting process. If a process is unable to change its state indefinitely because the resources requested by it are being used by another waiting process, then the system is said to be in a deadlock.

In a communications system, deadlocks occur mainly due to lost or corrupt signals rather than resource contention.

A deadlock situation can arise if and only if all of the following conditions hold simultaneously in a system:

These four conditions are known as the Coffman conditions from their first description in a 1971 article by Edward G. Coffman, Jr.

Most current operating systems cannot prevent deadlocks. When a deadlock occurs, different operating systems respond to them in different non-standard manners. Most approaches work by preventing one of the four Coffman conditions from occurring, especially the fourth one. Major approaches are as follows.

In this approach, it is assumed that a deadlock will never occur. This is also an application of the Ostrich algorithm. This approach was initially used by MINIX and UNIX. This is used when the time intervals between occurrences of deadlocks are large and the data loss incurred each time is tolerable.

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