Deadlock is a very important concept in operating systems, and it is often encountered in multi-threaded programming. This article will start with the concept and conditions of deadlock, and provide some methods to prevent deadlock.
1. Introduction to deadlock
A deadlock can occur when multiple threads simultaneously hog each other for resources that other threads need. A deadlock is a deadlock in which multiple threads compete for a shared resource. In the deadlock state, each thread is waiting for resources, and will not actively release the resources it already holds.
Second, the cause of deadlock
1. Mutually exclusive conditions
The first necessary condition for a deadlock is a mutual exclusion condition. That is, a resource can only be used by one process at a time. If the resource is held by one process, other processes cannot use the resource until one of the processes releases it. But if a resource requested by a process is being used by another process, the requesting process can only wait.
2. Request and Hold Conditions
The second necessary condition for deadlock is the request and hold condition. That is, the process already owns some resources, but makes a new resource request, and these new resources are occupied by other processes, so the requesting process is blocked. But it is unwilling to release the resources it already owns, leading to a situation of circular waiting.
3. Inalienable conditions
The third necessary condition for deadlock is the non-alienable condition. That is, certain resources cannot be forcibly deprived by other processes, and can only be released by the process that obtains the resource. If the process is unwilling to release the resource at this time, other processes requesting the resource will be blocked.
4. Loop waiting condition
The fourth necessary condition for deadlock is a circular wait condition. That is, a loop is formed between the processes involved in the deadlock, and each process is waiting for a resource held by the next process.
3. Prevent deadlock
In order to prevent deadlock, we need to destroy the above four necessary conditions, the specific method is as follows:
1. Destruction of mutual exclusion conditions
Mutual exclusion can be avoided by resource reuse or sharing. For example, some mutexes or condition variables can be designed to coordinate resource access among various processes.
2. Destruction request and hold condition
When a process applies for resources, it is required to apply for all the required resources at one time, instead of applying one by one, so that the request and hold conditions can be broken.
3. Violation of the inalienable condition
For some processes that are not allowed to be interrupted, preventive measures should be taken, such as backing up the system or using fault-tolerant mechanisms.
4. Destroy the loop wait condition
Circular wait conditions can be avoided by ordering resources or allocating resources hierarchically. For example, for resources required by multiple processes, a unique sequence number can be assigned to each resource, and the sequence numbers of processes applying for resources must be in ascending order.