People sometimes ask me if there is a cheap-and-easy way to guarantee thread safety. For example, “if my method only reads and writes local variables and parameters, can I guarantee that my method is threadsafe?” Questions like that are dangerous because they are predicated on an incorrect assumption: that if every method of a program is “threadsafe”, whatever that means, then the entire program is “threadsafe”. I might not be entirely clear on what “threadsafe” means, but I do know one thing about it: thread safety is a property of entire programs, not of individual methods.

To illustrate why these sorts of questions are non-starters, today I present to you the world’s simplest deadlocking C# program:

class C { static C() { // Let's run the initialization on another thread! var thread = new System.Threading.Thread(Initialize); thread.Start(); thread.Join(); } static void Initialize() { } static void Main() { } }

(Thanks to my Roslyn compiler team colleague Neal Gafter for this example, which was adapted from his book Java Puzzlers .)

At first glance clearly ever method of this incredibly simple program is “threadsafe”. There is only a single variable anywhere in the program; it is local, is written once, is written before it is read, is read from the same thread it was written on, and is guaranteed to be atomic. There are apparently no locks anywhere in the program, and so there are no lock ordering inversions. Two of the three methods are empty. And yet this program deadlocks with 100% certainty; the program “globally” is clearly not threadsafe, despite all those nice “local” properties. You can build a hollow house out of solid bricks; so too you can build a deadlocking program out of threadsafe methods.

The reason why this deadlocks is a consequence of the rules for static constructors in C#; the important rule is that a static constructor runs exactly zero or one times, and runs before a static method call or instance creation in its type. Therefore the static constructor of C must run to completion before Main starts. The CLR notes that C ‘s static constructor is “in flight” on the main thread and calls it. The static constructor then starts up a new thread. When that thread starts, the CLR sees that a static method is about to be called on a type whose static constructor is “in flight” another thread. It immediately blocks the new thread so that the Initialize method will not start until the main thread finishes running the class constructor. The main thread blocks itself waiting for the new thread to complete, and now we have two threads each waiting for the other to complete.

Next time on FAIC: We’re opening up the new Coverity office in Seattle! After which, we’ll take a closer look at the uses and abuses of the static constructor.