double-checked-locking

问题 I'm new to PHP, so to get started I've decided to implement a singleton. While I am able to recreate the singleton pattern in php, but I am not sure how to implement double-checked locking. Is that even possible/needed in PHP. I have read somewhere that PHP is not multithreaded? Can someone confirm that? If it is multithreaded, can someone explain to me how lock() or synchronize() work in PHP? Thanks, Henry 回答1: Share-nothing Architecture PHP has a Share-nothing Architecture: Like HTTP, each

问题 The fallowing clause is taken from jetbrains.net After reading this and some other articles on the web, I still don't understand how is it possible to return null, after the first thread go in to the lock. Some one that does understand it can please help me and explain it in more humanized way? "Consider the following piece of code: public class Foo { private static Foo instance; private static readonly object padlock = new object(); public static Foo Get() { if (instance == null) { lock

问题 In this link i found the singleton instantiation as below: public static Singleton getInstanceDC() { if (_instance == null) { // Single Checked (1) synchronized (Singleton.class) { if (_instance == null) { // Double checked _instance = new Singleton(); } } } return _instance; } I am not getting the point of single check ie (1) . Whats its use here any way the single thread will be checking the instance inside synchronized block , so what is point of using the first check? 回答1: Consider that

问题 that's what I have: a Windows Service C# multithreaded the service uses a Read-Write-Lock (multiple reads at one time, writing blocks other reading/writing threads) a simple, self-written DB C++ small enough to fit into memory big enough not wanting to load it on startup (e.g. 10GB) read-performance is very important writing is less important tree structure informations held in tree nodes are stored in files for faster performance, the files are loaded only the first time they are used and

问题 This question already has answers here : Java double checked locking (11 answers) Closed 5 years ago . There's a common belief and multiple sources (including wiki) that claim this idiom to be an anti-pattern. What are the arguments against using it in production code given the correct implementation is used (for example, using volatile ) What are the appropriate alternatives for implementing lazy initialization in a multithreaded environment ? Locking the whole method may become a bottleneck

问题 Recently I have been refactoring some of my C# code and I found a few double-checked locking practices taking place. I didn't know it was a bad practice back then and I really want to get rid of it. The problem is that I have a class that should be lazily initialized and frequently accessed by lots of threads. I also do not want to move the initialization to a static initializer, because I am planning to use a weak reference to keep the initialized object from staying too long in the memory.

问题 void undefined_behaviour_with_double_checked_locking() { if(!resource_ptr) #1 { std::lock_guard<std::mutex> lk(resource_mutex); #2 if(!resource_ptr) #3 { resource_ptr.reset(new some_resource); #4 } } resource_ptr->do_something(); #5 } if a thread sees the pointer written by another thread, it might not see the newly-created instance of some_resource, resulting in the call to do_something() operating on incorrect values. This is an example of the type of race condition defined as a data race

问题 Why is the pattern considered broken? It looks fine to me? Any ideas? public static Singleton getInst() { if (instace == null) createInst(); return instace; } private static synchronized createInst() { if (instace == null) { instace = new Singleton(); } } 回答1: It looks okay at first glance, but this technique has many subtle problems and should usually be avoided. For example, consider the following sequence of events: Thread A notices that the value is not initialized, so it obtains the lock

问题 In the examples mentioned for Out-of-order writes for double-checked locking scenarios (ref: IBM article & Wikipedia Article) I could not understand the simple reason of why Thread1 would come of out synchronized block before the constructor is fully initialized. As per my understanding, creating "new" and the calling constructor should execute in-sequence and the synchronized lock should not be release till all the work in not completed. Please let me know what I am missing here. 回答1: The

问题 Checkstyle reports this code as "The double-checked locking idiom is broken", but I don't think that my code actually is affected by the problems with double-checked locking. The code is supposed to create a row in a database if a row with that id doesn't exist. It runs in a multi-threaded environment and I want to avoid the primary-key-exists SQL-exceptions. The pseudo-code: private void createRow(int id) { Row row = dao().fetch(id); if (row == null) { synchronized (TestClass.class) { row =