Editing Deadlock (computer science) (section)

===Prevention===
{{main|Deadlock prevention algorithms}}
[[File:Avoiding deadlock.gif|380px|thumbnail|right|
(A) Two processes competing for one resource, following a first-come, first-served policy.

(B) Deadlock occurs when both processes lock the resource simultaneously.

(C) The deadlock can be ''resolved'' by breaking the symmetry of the locks.

(D) The deadlock can be ''prevented'' by breaking the symmetry of the locking mechanism.]]
Deadlock prevention works by preventing one of the four Coffman conditions from occurring.
* Removing the ''mutual exclusion'' condition means that no process will have exclusive access to a resource. This proves impossible for resources that cannot be [[Spooling|spooled]]. But even with spooled resources, the deadlock could still occur. Algorithms that avoid mutual exclusion are called [[non-blocking synchronization]] algorithms.
* The ''hold and wait'' or ''resource holding'' conditions may be removed by requiring processes to request all the resources they will need before starting up (or before embarking upon a particular set of operations). This advance knowledge is frequently difficult to satisfy and, in any case, is an inefficient use of resources. Another way is to require processes to request resources only when it has none; First, they must release all their currently held resources before requesting all the resources they will need from scratch. This too is often impractical. It is so because resources may be allocated and remain unused for long periods. Also, a process requiring a popular resource may have to wait indefinitely, as such a resource may always be allocated to some process, resulting in [[resource starvation]].<ref>{{cite book|last=Silberschatz|first=Abraham|url=https://books.google.com/books?id=WjvX0HmVTlMC&q=deadlock+operating+systems|publisher=Wiley-India|year=2006|title=Operating System Principles|edition=7|page=244|isbn=9788126509621|access-date=16 October 2020|archive-date=18 April 2021|archive-url=https://web.archive.org/web/20210418013932/https://books.google.com/books?id=WjvX0HmVTlMC&q=deadlock+operating+systems|url-status=live}}</ref> (These algorithms, such as [[serializing tokens]], are known as the ''all-or-none algorithms''.)
* The ''no [[Preemption (computing)|preemption]]'' condition may also be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or [[thrashing (computer science)|thrashing]] may occur. However, the inability to enforce preemption may interfere with a ''priority'' algorithm. Preemption of a "locked out" resource generally implies a [[Rollback (data management)|rollback]], and is to be avoided since it is very costly in overhead. Algorithms that allow preemption include [[lock-free and wait-free algorithms]] and [[optimistic concurrency control]]. If a process holding some resources and requests for some another resource(s) that cannot be immediately allocated to it, the condition may be removed by releasing all the currently being held resources of that process. 
* The final condition is the ''circular wait'' condition. Approaches that avoid circular waits include disabling interrupts during critical sections and using a hierarchy to determine a [[partial order]]ing of resources. If no obvious hierarchy exists, even the memory address of resources has been used to determine ordering and resources are requested in the increasing order of the enumeration.<ref name="os_galvin"/> [[Dining philosophers problem#Resource hierarchy solution|Dijkstra's solution]] can also be used.