死锁

死锁 | Deadlock

定义 | Definition

死锁是一种状态，其中两个或多个进程或线程因互相等待对方释放资源而无法继续执行。

Deadlock is a state in which two or more processes or threads are unable to proceed because each is waiting for the other to release resources.

死锁的必要条件 | Necessary Conditions for Deadlock

1. 互斥条件 | Mutual Exclusion

   • 至少有一个资源必须是不可共享的，即同一时间只能被一个进程使用。
   • At least one resource must be held in a non-shareable mode, meaning only one process can use the resource at a time.

2. 占有并等待条件 | Hold and Wait

   • 一个进程已经持有至少一个资源，并且正在等待获取其他资源，而这些资源正被其他进程持有。
   • A process holding at least one resource is waiting to acquire additional resources that are currently being held by other processes.

3. 不剥夺条件 | No Preemption

   • 资源不能被强制性地从进程中剥夺；资源只能由进程自己释放。
   • Resources cannot be forcibly removed from the processes holding them; they can only be released voluntarily by the process.

4. 环路等待条件 | Circular Wait

   • 存在一个进程集合 $\{P_1,P_2,\dots ,P_n\}$，其中 $P_1$ 正在等待 $P_2$ 持有的资源，$P_2$ 正在等待 $P_3$ 持有的资源，…，$P_n$ 正在等待 $P_1$ 持有的资源。
   • A set of processes $\{P_1,P_2,\dots ,P_n\}$ exists such that $P_1$ is waiting for a resource held by $P_2$, $P_2$ is waiting for a resource held by $P_3$, …, and $P_n$ is waiting for a resource held by $P_1$.

死锁预防、避免、检测和恢复 | Deadlock Prevention, Avoidance, Detection, and Recovery

死锁预防 | Deadlock Prevention

通过破坏死锁的必要条件来防止死锁的发生。

Prevent deadlocks by ensuring that at least one of the necessary conditions cannot hold.

1. 破坏互斥条件 | Breaking Mutual Exclusion

   • 尽可能将资源设计为可共享。
   • Design resources to be shareable wherever possible.

2. 破坏占有并等待条件 | Breaking Hold and Wait

   • 要求进程在进入临界区之前请求所有需要的资源。
   • Require processes to request all the resources they need before entering the critical section.

3. 破坏不剥夺条件 | Breaking No Preemption

   • 如果一个进程请求的资源无法立即获得，要求它释放所有已持有的资源，并在稍后重新请求。
   • If a process requests a resource that is not immediately available, require it to release all held resources and request again later.

4. 破坏环路等待条件 | Breaking Circular Wait

   • 对所有资源排序，并要求进程按顺序请求资源。
   • Impose a total ordering of all resources and require processes to request resources in an increasing order of enumeration.

死锁避免 | Deadlock Avoidance

在资源分配时进行动态检查，以确保不会进入死锁状态。

Dynamically check resource allocation to ensure that a system does not enter a deadlock state.

银行家算法 | Banker’s Algorithm

银行家算法是一种著名的死锁避免算法，通过模拟资源分配，确保系统始终处于安全状态。

The Banker’s Algorithm is a well-known deadlock avoidance algorithm that simulates resource allocation to ensure the system remains in a safe state.

1. 初始化 | Initialization

   • 每个进程声明最大资源需求。
   • Each process declares its maximum resource needs.

2. 资源请求 | Resource Request

   • 进程请求资源时，系统检查资源是否可用并判断分配后是否处于安全状态。
   • When a process requests resources, the system checks if they are available and whether allocation will keep the system in a safe state.

3. 安全状态 | Safe State

   • 如果系统处于安全状态，则分配资源；否则，进程必须等待。
   • If the system is in a safe state, allocate the resources; otherwise, the process must wait.

死锁检测和恢复 | Deadlock Detection and Recovery

允许死锁发生，但需要一种机制来检测并恢复。

Allow deadlocks to occur but provide a mechanism to detect and recover from them.

死锁检测 | Deadlock Detection

使用图算法检测系统中的死锁。

Use graph algorithms to detect deadlocks in the system.

1. 资源分配图 | Resource Allocation Graph

   • 将进程和资源表示为图中的节点，边表示资源的分配和请求。
   • Represent processes and resources as nodes in a graph, with edges indicating resource allocation and requests.

2. 检测算法 | Detection Algorithm

   • 寻找图中的环，环的存在表示死锁。
   • Look for cycles in the graph; the presence of a cycle indicates a deadlock.

# Example in Python
def detect_deadlock(resource_allocation_graph):
    # Detect cycles in the resource allocation graph
      def dfs(node, visited, stack):
         visited[node] = True
         stack[node] = True
         for neighbor in resource_allocation_graph[node]:
               if not visited[neighbor]:
                  if dfs(neighbor, visited, stack):
                     return True
               elif stack[neighbor]:
                  return True
         stack[node] = False
         return False

死锁恢复 | Deadlock Recovery

一旦检测到死锁，采取措施恢复系统。

Once a deadlock is detected, take measures to recover the system.

1. 资源抢占 | Resource Preemption

   • 从某些进程中抢占资源并分配给其他进程。
   • Preempt resources from some processes and allocate them to others.

2. 终止进程 | Process Termination

   • 终止部分或全部死锁进程。
   • Terminate some or all of the deadlocked processes.

3. 回滚 | Rollback

   • 回滚进程到先前的状态，释放资源。
   • Roll back processes to a previous state and release resources.

总结 | Summary

死锁是一种严重的系统问题，通过理解和应用预防、避免、检测和恢复策略，可以有效管理和解决死锁问题。

Deadlock is a serious system issue that can be effectively managed and resolved by understanding and applying strategies for prevention, avoidance, detection, and recovery.