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8.4. Multi-threading in Wine

This section will assume you understand the basics of multithreading. If not there are plenty of good tutorials available on the net to get you started.

Threading in Wine is somewhat complex due to several factors. The first is the advanced level of multithreading support provided by Windows - there are far more threading related constructs available in Win32 than the Linux equivalent (pthreads). The second is the need to be able to map Win32 threads to native Linux threads which provides us with benefits like having the kernel schedule them without our intervention. While it's possible to implement threading entirely without kernel support, doing so is not desirable on most platforms that Wine runs on.

8.4.1. Threading support in Win32

Win32 is an unusually thread friendly API. Not only is it entirely thread safe, but it provides many different facilities for working with threads. These range from the basics such as starting and stopping threads, to the extremely complex such as injecting threads into other processes and COM inter-thread marshalling.

One of the primary challenges of writing Wine code therefore is ensuring that all our DLLs are thread safe, free of race conditions and so on. This isn't simple - don't be afraid to ask if you aren't sure whether a piece of code is thread safe or not!

Win32 provides many different ways you can make your code thread safe however the most common are critical section and the interlocked functions. Critical sections are a type of mutex designed to protect a geographic area of code. If you don't want multiple threads running in a piece of code at once, you can protect them with calls to EnterCriticalSection() and LeaveCriticalSection(). The first call to EnterCriticalSection() by a thread will lock the section and continue without stopping. If another thread calls it then it will block until the original thread calls LeaveCriticalSection() again.

It is therefore vitally important that if you use critical sections to make some code thread-safe, that you check every possible codepath out of the code to ensure that any held sections are left. Code like this:

if (res != ERROR_SUCCESS) return res;

is extremely suspect in a function that also contains a call to EnterCriticalSection(). Be careful.

If a thread blocks while waiting for another thread to leave a critical section, you will see an error from the RtlpWaitForCriticalSection() function, along with a note of which thread is holding the lock. This only appears after a certain timeout, normally a few seconds. It's possible the thread holding the lock is just being really slow which is why Wine won't terminate the app like a non-checked build of Windows would, but the most common cause is that for some reason a thread forgot to call LeaveCriticalSection(), or died while holding the lock (perhaps because it was in turn waiting for another lock). This doesn't just happen in Wine code: a deadlock while waiting for a critical section could be due to a bug in the app triggered by a slight difference in the emulation.

Another popular mechanism available is the use of functions like InterlockedIncrement() and InterlockedExchange(). These make use of native CPU abilities to execute a single instruction while ensuring any other processors on the system cannot access memory, and allow you to do common operations like add/remove/check a variable in thread-safe code without holding a mutex. These are useful for reference counting especially in free-threaded (thread safe) COM objects.

Finally, the usage of TLS slots are also popular. TLS stands for thread-local storage, and is a set of slots scoped local to a thread which you can store pointers in. Look on MSDN for the TlsAlloc() function to learn more about the Win32 implementation of this. Essentially, the contents of a given slot will be different in each thread, so you can use this to store data that is only meaningful in the context of a single thread. On recent versions of Linux the __thread keyword provides a convenient interface to this functionality - a more portable API is exposed in the pthread library. However, these facilities are not used by Wine, rather, we implement Win32 TLS entirely ourselves.

8.4.2. The Win32 thread environment

All Win32 code, whether from a native EXE/DLL or in Wine itself, expects certain constructs to be present in its environment. This section explores what those constructs are and how Wine sets them up. The lack of this environment is one thing that makes it hard to use Wine code directly from standard Linux applications - in order to interact with Win32 code a thread must first be "adopted" by Wine.

The first thing Win32 code requires is the TEB or "Thread Environment Block". This is an internal (undocumented) Windows structure associated with every thread which stores a variety of things such as TLS slots, a pointer to the threads message queue, the last error code and so on. You can see the definition of the TEB in include/thread.h, or at least what we know of it so far. Being internal and subject to change, the layout of the TEB has had to be reverse engineered from scratch.

A pointer to the TEB is stored in the %fs register and can be accessed using NtCurrentTeb() from within Wine code. %fs actually stores a selector, and setting it therefore requires modifying the processes local descriptor table (LDT) - the code to do this is in lib/wine/ldt.c.

The TEB is required by nearly all Win32 code run in the Wine environment, as any wineserver RPC will use it, which in turn implies that any code which could possibly block for instance by using a critical section) needs it. The TEB also holds the SEH exception handler chain as the first element, so if disassembling you see code like this:

movl %esp, %fs:0

... then you are seeing the program set up an SEH handler frame. All threads must have at least one SEH entry, which normally points to the backstop handler which is ultimately responsible for popping up the all-too-familiar "This program has performed an illegal operation and will be terminated" message. On Wine we just drop straight into the debugger. A full description of SEH is out of the scope of this section, however there are some good articles in MSJ if you are interested.

All Win32-aware threads must have a wineserver connection. Many different APIs require the ability to communicate with the wineserver. In turn, the wineserver must be aware of Win32 threads in order to be able to accurately report information to other parts of the program and do things like route inter-thread messages, dispatch APCs (asynchronous procedure calls) and so on. Therefore a part of thread initialization is initializing the thread server-side. The result is not only correct information in the server, but a set of file descriptors the thread can use to communicate with the server - the request fd, reply fd and wait fd (used for blocking).