1 <chapter id="threading">
2 <title>Multi-threading in Wine</title>
5 This section will assume you understand the basics of multithreading. If not there are plenty of
6 good tutorials available on the net to get you started.
10 Threading in Wine is somewhat complex due to several factors. The first is the advanced level of
11 multithreading support provided by Windows - there are far more threading related constructs available
12 in Win32 than the Linux equivalent (pthreads). The second is the need to be able to map Win32 threads
13 to native Linux threads which provides us with benefits like having the kernel schedule them without
14 our intervention. While it's possible to implement threading entirely without kernel support, doing so
15 is not desirable on most platforms that Wine runs on.
19 <title> Threading support in Win32 </title>
22 Win32 is an unusually thread friendly API. Not only is it entirely thread safe, but it provides
23 many different facilities for working with threads. These range from the basics such as starting
24 and stopping threads, to the extremely complex such as injecting threads into other processes and
25 COM inter-thread marshalling.
29 One of the primary challenges of writing Wine code therefore is ensuring that all our DLLs are
30 thread safe, free of race conditions and so on. This isn't simple - don't be afraid to ask if
31 you aren't sure whether a piece of code is thread safe or not!
35 Win32 provides many different ways you can make your code thread safe however the most common
36 are <emphasis>critical section</emphasis> and the <emphasis>interlocked functions</emphasis>.
37 Critical sections are a type of mutex designed to protect a geographic area of code. If you don't
38 want multiple threads running in a piece of code at once, you can protect them with calls to
39 EnterCriticalSection and LeaveCriticalSection. The first call to EnterCriticalSection by a thread
40 will lock the section and continue without stopping. If another thread calls it then it will block
41 until the original thread calls LeaveCriticalSection again.
45 It is therefore vitally important that if you use critical sections to make some code thread-safe,
46 that you check every possible codepath out of the code to ensure that any held sections are left.
50 <programlisting> if (res != ERROR_SUCCESS) return res; </programlisting>
53 is extremely suspect in a function that also contains a call to EnterCriticalSection. Be careful.
57 If a thread blocks while waiting for another thread to leave a critical section, you will
58 see an error from the RtlpWaitForCriticalSection function, along with a note of which
59 thread is holding the lock. This only appears after a certain timeout, normally a few
60 seconds. It's possible the thread holding the lock is just being really slow which is why
61 Wine won't terminate the app like a non-checked build of Windows would, but the most
62 common cause is that for some reason a thread forgot to call LeaveCriticalSection, or died
63 while holding the lock (perhaps because it was in turn waiting for another lock). This
64 doesn't just happen in Wine code: a deadlock while waiting for a critical section could
65 be due to a bug in the app triggered by a slight difference in the emulation.
69 Another popular mechanism available is the use of functions like InterlockedIncrement and
70 InterlockedExchange. These make use of native CPU abilities to execute a single
71 instruction while ensuring any other processors on the system cannot access memory, and
72 allow you to do common operations like add/remove/check a variable in thread-safe code
73 without holding a mutex. These are useful for reference counting especially in
74 free-threaded (thread safe) COM objects.
78 Finally, the usage of TLS slots are also popular. TLS stands for thread-local storage, and is
79 a set of slots scoped local to a thread which you can store pointers in. Look on MSDN for the
80 TlsAlloc function to learn more about the Win32 implementation of this. Essentially, the
81 contents of a given slot will be different in each thread, so you can use this to store data
82 that is only meaningful in the context of a single thread. On recent versions of Linux the
83 __thread keyword provides a convenient interface to this functionality - a more portable API
84 is exposed in the pthread library. However, these facilities is not used by Wine, rather, we
85 implement Win32 TLS entirely ourselves.
90 <title> SysLevels </title>
93 SysLevels are an undocumented Windows-internal thread-safety system. They are basically
94 critical sections which must be taken in a particular order. The mechanism is generic but
95 there are always three syslevels: level 1 is the Win16 mutex, level 2 is the USER mutex
96 and level 3 is the GDI mutex.
100 When entering a syslevel, the code (in dlls/kernel/syslevel.c) will check that a
101 higher syslevel is not already held and produce an error if so. This is because it's not
102 legal to enter level 2 while holding level 3 - first, you must leave level 3.
106 Throughout the code you may see calls to _ConfirmSysLevel() and _CheckNotSysLevel(). These
107 functions are essentially assertions about the syslevel states and can be used to check
108 that the rules have not been accidentally violated. In particular, _CheckNotSysLevel()
109 will break (probably into the debugger) if the check fails. If this happens the solution
110 is to get a backtrace and find out, by reading the source of the wine functions called
111 along the way, how Wine got into the invalid state.
117 <title> POSIX threading vs kernel threading </title>
120 Wine runs in one of two modes: either pthreads (posix threading) or kthreads (kernel
121 threading). This section explains the differences between them. The one that is used is
122 automatically selected on startup by a small test program which then execs the correct
123 binary, either wine-kthread or wine-pthread. On NPTL-enabled systems pthreads will be
124 used, and on older non-NPTL systems kthreads is selected.
128 Let's start with a bit of history. Back in the dark ages when Wines threading support was
129 first implemented a problem was faced - Windows had much more capable threading APIs than
130 Linux did. This presented a problem - Wine works either by reimplementing an API entirely
131 or by mapping it onto the underlying systems equivalent. How could Win32 threading be
132 implemented using a library which did not have all the neeed features? The answer, of
133 course, was that it couldn't be.
137 On Linux the pthreads interface is used to start, stop and control threads. The pthreads
138 library in turn is based on top of so-called "kernel threads" which are created using the
139 clone(2) syscall. Pthreads provides a nicer (more portable) interface to this
140 functionality and also provides APIs for controlling mutexes. There is a
141 <ulink url="http://www.llnl.gov/computing/tutorials/pthreads/">
142 good tutorial on pthreads </ulink> available if you want to learn more.
146 As pthreads did not provide the necessary semantics to implement Win32 threading, the
147 decision was made to implement Win32 threading on top of the underlying kernel threads by
148 using syscalls like clone directly. This provided maximum flexibility and allowed a
149 correct implementation but caused some bad side effects. Most notably, all the userland
150 Linux APIs assumed that the user was utilising the pthreads library. Some only enabled
151 thread safety when they detected that pthreads was in use - this is true of glibc, for
152 instance. Worse, pthreads and pure kernel threads had strange interactions when run in
153 the same process yet some libraries used by Wine used pthreads internally. Throw in
154 source code porting using WineLib - where you have both UNIX and Win32 code in the same
155 process - and chaos was the result.
159 The solution was simple yet ingenius: Wine would provide its own implementation of the pthread
160 library <emphasis>inside</emphasis> its own binary. Due to the semantics of ELF symbol
161 scoping, this would cause Wines own implementations to override any implementation loaded
162 later on (like the real libpthread.so). Therefore, any calls to the pthread APIs in
163 external libraries would be linked to Wines instead of the systems pthreads library, and
164 Wine implemented pthreads by using the standard Windows threading APIs it in turn
169 As a result, libraries that only became thread-safe in the presence of a loaded pthreads
170 implementation would now do so, and any external code that used pthreads would actually
171 end up creating Win32 threads that Wine was aware of and controlled. This worked quite
172 nicely for a long time, even though it required doing some extremely un-kosher things like
173 overriding internal libc structures and functions. That is, it worked until NPTL was
174 developed at which point the underlying thread implementation on Linux changed
179 The fake pthread implementation can be found in loader/kthread.c, which is used to
180 produce to wine-kthread binary. In contrast, loader/pthread.c produces the wine-pthread
181 binary which is used on newer NPTL systems.
185 NPTL is a new threading subsystem for Linux that hugely improves its performance and
186 flexibility. By allowing threads to become much more scalable and adding new pthread
187 APIs, NPTL made Linux competitive with Windows in the multi-threaded world. Unfortunately
188 it also broke many assumptions made by Wine (as well as other applications such as the
189 Sun JVM and RealPlayer) in the process.
193 There was, however, some good news. NPTL made Linux threading powerful enough
194 that Win32 threads could now be implemented on top of pthreads like any other normal
195 application. There would no longer be problems with mixing win32-kthreads and pthreads
196 created by external libraries, and no need to override glibc internals. As you can see
197 from the relative sizes of the loader/kthread.c and loader/pthread.c files, the
198 difference in code complexity is considerable. NPTL also made several other semantic
199 changes to things such as signal delivery so changes were required in many different
204 On non-Linux systems the threading interface is typically not powerful enough to
205 replicate the semantics Win32 applications expect and so kthreads with the
206 pthread overrides are used.
211 <title> The Win32 thread environment </title>
214 All Win32 code, whether from a native EXE/DLL or in Wine itself, expects certain constructs to
215 be present in its environment. This section explores what those constructs are and how Wine
216 sets them up. The lack of this environment is one thing that makes it hard to use Wine code
217 directly from standard Linux applications - in order to interact with Win32 code a thread
218 must first be "adopted" by Wine.
222 The first thing Win32 code requires is the <emphasis>TEB</emphasis> or "Thread Environment
223 Block". This is an internal (undocumented) Windows structure associated with every thread
224 which stores a variety of things such as TLS slots, a pointer to the threads message queue,
225 the last error code and so on. You can see the definition of the TEB in include/thread.h, or
226 at least what we know of it so far. Being internal and subject to change, the layout of the
227 TEB has had to be reverse engineered from scratch.
231 A pointer to the TEB is stored in the %fs register and can be accessed using NtCurrentTeb()
232 from within Wine code. %fs actually stores a selector, and setting it therefore requires
233 modifying the processes local descriptor table (LDT) - the code to do this is in lib/wine/ldt.c.
237 The TEB is required by nearly all Win32 code run in the Wine environment, as any wineserver
238 RPC will use it, which in turn implies that any code which could possibly block (for instance
239 by using a critical section) needs it. The TEB also holds the SEH exception handler chain as
240 the first element, so if when disassembling you see code like this:
243 <programlisting> movl %esp, %fs:0 </programlisting>
246 ... then you are seeing the program set up an SEH handler frame. All threads must have at
247 least one SEH entry, which normally points to the backstop handler which is ultimately
248 responsible for popping up the all-too-familiar "This program has performed an illegal
249 operation and will be terminated" message. On Wine we just drop straight into the debugger.
250 A full description of SEH is out of the scope of this section, however there are some good
251 articles in MSJ if you are interested.
255 All Win32-aware threads must have a wineserver connection. Many different APIs
256 require the ability to communicate with the wineserver. In turn, the wineserver must be aware
257 of Win32 threads in order to be able to accurately report information to other parts of the
258 program and do things like route inter-thread messages, dispatch APCs (asynchronous procedure
259 calls) and so on. Therefore a part of thread initialization is initializing the thread
260 serverside. The result is not only correct information in the server, but a set of file
261 descriptors the thread can use to communicate with the server - the request fd, reply fd and
262 wait fd (used for blocking).
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