8.4. Multi-threading in Wine

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. POSIX threading vs. kernel threading

Wine runs in one of two modes: either pthreads (posix threading) or kthreads (kernel threading). This section explains the differences between them. The one that is used is automatically selected on startup by a small test program which then execs the correct binary, either wine-kthread or wine-pthread. On NPTL-enabled systems pthreads will be used, and on older non-NPTL systems kthreads is selected.

Let's start with a bit of history. Back in the dark ages when Wine's threading support was first implemented a problem was faced - Windows had much more capable threading APIs than Linux did. This presented a problem - Wine works either by reimplementing an API entirely or by mapping it onto the underlying systems equivalent. How could Win32 threading be implemented using a library which did not have all the needed features? The answer, of course, was that it couldn't be.

On Linux the pthreads interface is used to start, stop and control threads. The pthreads library in turn is based on top of so-called "kernel threads" which are created using the clone(2) syscall. Pthreads provides a nicer (more portable) interface to this functionality and also provides APIs for controlling mutexes. There is a good tutorial on pthreads available if you want to learn more.

As pthreads did not provide the necessary semantics to implement Win32 threading, the decision was made to implement Win32 threading on top of the underlying kernel threads by using syscalls like clone() directly. This provided maximum flexibility and allowed a correct implementation but caused some bad side effects. Most notably, all the userland Linux APIs assumed that the user was utilising the pthreads library. Some only enabled thread safety when they detected that pthreads was in use - this is true of glibc, for instance. Worse, pthreads and pure kernel threads had strange interactions when run in the same process yet some libraries used by Wine used pthreads internally. Throw in source code porting using Winelib - where you have both UNIX and Win32 code in the same process - and chaos was the result.

The solution was simple yet ingenious: Wine would provide its own implementation of the pthread library inside its own binary. Due to the semantics of ELF symbol scoping, this would cause Wine's own implementation to override any implementation loaded later on (like the real libpthread.so). Therefore, any calls to the pthread APIs in external libraries would be linked to Wine's instead of the system's pthreads library, and Wine implemented pthreads by using the standard Windows threading APIs it in turn implemented itself.

As a result, libraries that only became thread-safe in the presence of a loaded pthreads implementation would now do so, and any external code that used pthreads would actually end up creating Win32 threads that Wine was aware of and controlled. This worked quite nicely for a long time, even though it required doing some extremely un-kosher things like overriding internal libc structures and functions. That is, it worked until NPTL was developed at which point the underlying thread implementation on Linux changed dramatically.

The fake pthread implementation can be found in loader/kthread.c, which is used to produce the wine-kthread binary. In contrast, loader/pthread.c produces the wine-pthread binary which is used on newer NPTL systems.

NPTL is a new threading subsystem for Linux that hugely improves its performance and flexibility. By allowing threads to become much more scalable and adding new pthread APIs, NPTL made Linux competitive with Windows in the multi-threaded world. Unfortunately it also broke many assumptions made by Wine (as well as other applications such as the Sun JVM and RealPlayer) in the process.

There was, however, some good news. NPTL made Linux threading powerful enough that Win32 threads could now be implemented on top of pthreads like any other normal application. There would no longer be problems with mixing win32-kthreads and pthreads created by external libraries, and no need to override glibc internals. As you can see from the relative sizes of the loader/kthread.c and loader/pthread.c files, the difference in code complexity is considerable. NPTL also made several other semantic changes to things such as signal delivery so changes were required in many different places in Wine.

On non-Linux systems the threading interface is typically not powerful enough to replicate the semantics Win32 applications expect and so kthreads with the pthread overrides are used.

8.4.3. 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).