Computer Deadlock

A deadlock occurs in a concurrent system when multiple processes are unable to proceed because they are mutually waiting for each other to release resources. The occurrence of deadlocks is primarily caused by two common factors: insufficient system resources and improper process progression order.

  1. Insufficient System Resources: Deadlocks can occur when system resources are insufficient, also known as resource contention. When multiple processes simultaneously request limited resources within the system, and each process holds some resources while waiting for others to be released, a deadlock may occur. For example, two processes hold Resource A and Resource B respectively, and each process requires the resource held by the other to continue execution. If these two processes simultaneously request the resources held by the other, they will wait for each other to release the resources, leading to a deadlock.

  2. Improper Process Progression Order: Deadlocks can also occur when the progression order of processes is inappropriate. When multiple processes request and release resources in an improper sequence, a deadlock may result. In this scenario, the occurrence of a deadlock is not due to insufficient system resources but rather due to mutual dependencies and ordering between processes that lead to circular waiting. For example, Process A holds Resource X and waits for Resource Y, while Process B holds Resource Y and waits for Resource X. If the request and release order of these two processes is improper, it will result in a deadlock.

Deadlock is a serious issue in concurrent systems, leading to waste of system resources and preventing processes from executing normally. To avoid the occurrence of deadlocks, the following measures can be taken: optimizing resource allocation strategies and scheduling strategies to avoid resource contention and circular waiting; introducing deadlock detection and recovery mechanisms to detect deadlocks in a timely manner and take actions to resolve them; designing reasonable resource management algorithms, such as the Banker's Algorithm, to ensure the safety and reliability of resource allocation.

The Four Necessary Conditions for Deadlock

  • Mutual Exclusion Condition: A process uses the acquired resources exclusively; at any given moment, a resource is occupied by only one process.
  • Hold and Wait Condition: When a process requests resources but cannot be satisfied and blocks itself, it does not release the resources already allocated to it. This condition is also referred to as the partial allocation condition.
  • No Preemption (Deprivation) Condition: Resources acquired by a process cannot be preempted by other processes before they are fully utilized; they can only be released by the process occupying them.
  • Circular Wait Condition: A set of processes forms a circular wait chain, where each process in the chain is waiting for a resource held by the next process in the chain.

Deadlock Prevention

Deadlock prevention aims to prevent deadlocks from occurring or reduce the likelihood of their occurrence. Below are three common methods for preventing deadlocks:

  1. Violating the "Hold and Wait" Condition: One condition for deadlock is that a process continues to request other resources while holding some. To violate this condition, measures can be taken to require processes to acquire all needed resources at once upon starting execution, rather than requesting them one by one. If a process cannot acquire all resources at once, it should release the already acquired resources and wait until it can acquire all required resources at once.

  2. Violating the "No Preemption" Condition: Another condition for deadlock is that resources cannot be preempted; that is, once a process has obtained a resource, other processes cannot forcibly seize it from it. To violate this condition, a resource preemption mechanism can be introduced. When another process needs a resource that is currently occupied, it can seize the resource and temporarily deprive the owner of its usage rights. The owner of the deprived resource will be blocked until the resource is released and reallocated to it.

  3. Violating the "Circular Wait" Condition: The third condition for deadlock is the existence of circular waiting between processes, meaning each process is waiting for a resource held by the next process. To violate this condition, a resource ordering allocation can be introduced. By numbering or sorting resources, processes are required to request resources in a specific order, thereby avoiding the occurrence of circular waiting. Additionally, a resource pre-allocation strategy can be introduced so that each process can obtain all the resources it needs before starting execution.

These deadlock prevention methods can be implemented during the design and programming phases to reduce the occurrence of deadlocks. However, these methods may incur certain performance losses or limit resource availability. Therefore, in practical applications, suitable deadlock prevention methods should be selected by comprehensively considering specific system requirements and resource management strategies.

Deadlock Avoidance

Deadlock avoidance involves imposing fewer restrictions on resource allocation policies to prevent the occurrence of deadlocks without changing the inherent nature of the resources.

In other words, while deadlock prevention requires violating one of the four necessary conditions for deadlock, deadlock avoidance does not necessarily require violating any of the four necessary conditions; instead, it imposes a few additional restrictions on the resource allocation policy to avoid deadlocks.

Deadlock Detection and Resolution

Deadlock Detection

Deadlock detection is a method used to determine whether a deadlock exists within a system. Through deadlock detection, the existence of a deadlock can be identified in a timely manner so that appropriate measures can be taken to resolve it. The general process for deadlock detection is as follows:

  1. Constructing the Resource Allocation Graph: The first step in deadlock detection is constructing the resource allocation graph. This graph serves as a graphical representation describing the relationships between processes and resources within the system. Nodes in the graph represent processes and resources, while edges represent request and ownership relationships between processes and resources. The resource allocation graph can be either a directed or undirected graph, depending on the nature of the resource allocation relationship.

  2. Searching for Deadlock Cycles: In a resource allocation graph, deadlocks typically manifest as one or more cycles. By traversing the resource allocation graph, it is possible to search for the existence of any cyclic paths. If a cycle path is found, it implies the potential existence of a deadlock.

  3. Determining the Type of Deadlock: During the deadlock detection process, the type of deadlock can be further determined. Common types of deadlocks include resource deadlocks and process deadlocks. A resource deadlock refers to multiple processes mutually waiting for resources occupied by each other, whereas a process deadlock refers to multiple processes mutually waiting for each other's execution.

  4. Judging the Existence of Deadlock: If a deadlock cycle exists and cannot be resolved, it can be judged that a deadlock exists within the system. The deadlock detection algorithm will provide a corresponding judgment based on the type of deadlock and the detection results.

  5. Executing Deadlock Recovery Measures: Once a deadlock is confirmed to exist within the system, appropriate deadlock recovery measures must be taken. Common methods for resolving deadlocks include deadlock resolution, deadlock prevention, and deadlock avoidance. The specific measure adopted depends on the system's requirements and strategies.

It should be noted that the deadlock detection algorithm itself also consumes certain system resources; therefore, a suitable detection strategy should be selected based on the actual conditions and performance requirements of the system. Deadlock detection is typically used in long-running systems or systems with frequent resource allocation to ensure system stability and reliability.

Deadlock Theorem

Based on the simplification result of the resource allocation graph, it can be determined whether the system state corresponding to the resource allocation graph is a deadlock state.

A sufficient condition for the system state S at a certain moment being in a deadlock state is that and only if the resource allocation graph in state S cannot be completely simplified; this sufficient condition is known as the Deadlock Theorem.

Deadlock Detection Algorithm

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Deadlock Resolution

  • Revoke all deadlocked processes.
  • Roll back deadlocked processes to some checkpoint in their normal execution state and then restart all processes.
  • Revoke deadlocked processes one by one according to a certain order until no deadlock occurs.

Principle of Minimum Cost in Deadlock

  • Minimum CPU time consumed up to the point of deadlock discovery.
  • Minimum total amount of system resources obtained up to the point of deadlock discovery.
  • Minimum amount of output produced up to the point of deadlock discovery.
  • Lowest priority.
  • Longest estimated remaining time for the process.