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GWO wonders what the conditions are under which the lock-free version significantly outpeforms a simple critical section. Remember that switching to a lock-free algorithm should be guided by performance measurements. Switching from a simple algorithm to a complex one shouldn't be done unless you know that the simple algorithm is having trouble....

Adam Rosenfield pointed out that it is not possible to fail an asynchronous initialization; if you pass when completing an asynchronous initialization, the function fails with . (Serves me right for writing an article the night before it goes up.) A more correct version is therefore In other words, the pattern is as follows: While I'm her...

If you are using Visual Studio 2005 or later, then you don't need the weird function because Visual Studio 2005 and later automatically impose acquire semantics on reads from volatile locations. It also imposes release semantics on writes to volatile locations. In other words, you can replace the old function with the following: This is a g...

In addition to the loader lock, the message queue is another resource that people often forget to incorporate into their lock hierarchy. If your code runs on a UI thread, then it implicitly owns the message queue whenever it is running, because messages cannot be dispatched to a thread until it calls a message-retrieval function such as or . In o...

The last lock-free pattern for this week isn't actually lock-free, but it does run without blocking. The pattern for what I'll call try/commit/(hand off) is more complicated than the other patterns, so I'll start off by describing it in words rather than in code, because the code tends to make things more complicated. First, you take the state...

Suppose profiling reveals that a specific calculation is responsible for a significant portion of your CPU time, and instrumentation says that most of the time, it's just being asked to calculate the same thing over and over. A simple one-level cache would do the trick here. Of course, this isn't thread-safe, because if one thread is pre-empted...

A customer was looking for advice on this synchronization problem: We have a small amount of data that we need to share among multiple processes. One way to protect the data is to use a spin lock. However, that has potential for deadlock if the process which holds the spinlock doesn't get a chance to release it. For example, it might be suspend...

The singleton constructor pattern and the example we saw some time ago are really special cases of the more general pattern which I'll call try/commit/(try again). I don't know if this pattern has a real name, but that's what I'm calling it for today. The general form of this pattern goes like this: We calculate the desired new value based ...

Letting the smart people do the work for you.

A few days ago, I asked you to make an existing class multithread-safe. The class caches objects called which are indexed by a 32-bit ID. The cache is implemented as an array that dynamically resizes as more items are added to it. A naïve multithreaded version might use a slim reader-writer lock with shared access on reads, exclusive access...