# Introduction

Generically, a **resource** is something a process needs in order to proceed with its execution.

* **physical components of the computational system** (processor, memory, I/O devices, etc.).
* **common data** structures defined at the operating system level (PCT, communication channels, etc,) or among processes of a given application.

Resources can be:

* **preemptable** – if they can be withdraw from the processes that hold them.
  * ex: processor, memory regions used by a process address space.
* **non-preemptable** – if they can only be released by the processes that hold them.
  * ex: a file, a shared memory region that requires exclusive access for its manipulation.

For this topic, only non-preemptable resources are relevant.

## Illustrating deadlock

<figure><img src="/files/En409yXmkdGmyv2BE34m" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/AjPvLybYDrcjXlAH1AUq" alt=""><figcaption></figcaption></figure>

*P* needs *S* to proceed, which is on possession of *Q*.

*Q* needs *R* to proceed, which is on possession of *P*.

What are the conditions for the occurrence of deadlock?

## Necessary conditions for deadlock

It can be proved that when deadlock occurs 4 conditions are necessarily observed:

* It can be proved that when deadlock occurs 4 conditions are necessarily observed:
* **mutual exclusion** – only one process may use a resource at a time.
  * if another process requests it, it must wait until it is released.
* **hold and wait** – A process waits for some resources while holding others at the same time.
* **no preemption** – resources are non-preemptable.
  * only the process holding a resource can release it, after completing its task.
* **circular wait** – a set of waiting processes must exist such that each one is waiting for resources held by other processes in the set.
  * there are loops in the graph.

<figure><img src="/files/drX2wndP2P1S342L1bHm" alt=""><figcaption></figcaption></figure>

## IIlustrating with the philosopher dinner problem

<figure><img src="/files/49EBIHpFjltG9ySpono1" alt=""><figcaption></figcaption></figure>

5 philosophers are seated around a table, with food in from of them. To eat, every philosopher needs two forks, the ones at her/his left and right sides. Every philosopher alternates periods in which she/he medidates with periods in which she/he eats.

Modelling every philosopher as a **different process or thread** and the forks as resources, **design a solution for the problem**.


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