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<chapter id="threading">
<title>Multi-threading in Wine</title>
<para>
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.
</para>
<para>
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.
</para>
<sect1>
<title> Threading support in Win32 </title>
<para>
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.
</para>
<para>
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!
</para>
<para>
Win32 provides many different ways you can make your code thread safe however the most common
are <emphasis>critical section</emphasis> and the <emphasis>interlocked functions</emphasis>.
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.
</para>
<para>
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:
</para>
<programlisting> if (res != ERROR_SUCCESS) return res; </programlisting>
<para>
is extremely suspect in a function that also contains a call to EnterCriticalSection. Be careful.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
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 is not used by Wine, rather, we
implement Win32 TLS entirely ourselves.
</para>
</sect1>
<sect1>
<title> SysLevels </title>
<para>
SysLevels are an undocumented Windows-internal thread-safety system. They are basically
critical sections which must be taken in a particular order. The mechanism is generic but
there are always three syslevels: level 1 is the Win16 mutex, level 2 is the USER mutex
and level 3 is the GDI mutex.
</para>
<para>
When entering a syslevel, the code (in dlls/kernel/syslevel.c) will check that a
higher syslevel is not already held and produce an error if so. This is because it's not
legal to enter level 2 while holding level 3 - first, you must leave level 3.
</para>
<para>
Throughout the code you may see calls to _ConfirmSysLevel() and _CheckNotSysLevel(). These
functions are essentially assertions about the syslevel states and can be used to check
that the rules have not been accidentally violated. In particular, _CheckNotSysLevel()
will break (probably into the debugger) if the check fails. If this happens the solution
is to get a backtrace and find out, by reading the source of the wine functions called
along the way, how Wine got into the invalid state.
</para>
</sect1>
<sect1>
<title> POSIX threading vs kernel threading </title>
<para>
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.
</para>
<para>
Let's start with a bit of history. Back in the dark ages when Wines 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 neeed features? The answer, of
course, was that it couldn't be.
</para>
<para>
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
<ulink url="http://www.llnl.gov/computing/tutorials/pthreads/">
good tutorial on pthreads </ulink> available if you want to learn more.
</para>
<para>
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.
</para>
<para>
The solution was simple yet ingenius: Wine would provide its own implementation of the pthread
library <emphasis>inside</emphasis> its own binary. Due to the semantics of ELF symbol
scoping, this would cause Wines own implementations 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 Wines instead of the systems pthreads library, and
Wine implemented pthreads by using the standard Windows threading APIs it in turn
implemented itself.
</para>
<para>
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.
</para>
<para>
The fake pthread implementation can be found in loader/kthread.c, which is used to
produce to wine-kthread binary. In contrast, loader/pthread.c produces the wine-pthread
binary which is used on newer NPTL systems.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
</sect1>
<sect1>
<title> The Win32 thread environment </title>
<para>
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.
</para>
<para>
The first thing Win32 code requires is the <emphasis>TEB</emphasis> 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.
</para>
<para>
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.
</para>
<para>
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 when disassembling you see code like this:
</para>
<programlisting> movl %esp, %fs:0 </programlisting>
<para>
... 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.
</para>
<para>
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
serverside. 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).
</para>
</sect1>
</chapter>
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