Getting Started with QNX Neutrino - QNX Software Systems

More on synchronization
© 2009, QNX Software Systems GmbH & Co. KG.
(Used for access and testing)
MutexXYZ
x y z
CondvarXYZ
(Used for waiting and waking)
One-to-one mutex and condvar associations.
One interesting note. Since there is no checking, you can do things like associate one
set of variables with mutex “ABC,” and another set of variables with mutex “DEF,”
while associating both sets of variables with condvar “ABCDEF:”
(Used for access and testing)
MutexABC
MutexDEF
a b c d e f
CondvarABCDEF
(Used for waiting and waking)
Many-to-one mutex and condvar associations.
Additional Neutrino services
This is actually quite useful. Since the mutex is always to be used for “access and
testing,” this implies that I have to choose the correct mutex whenever I want to look at
a particular variable. Fair enough — if I’m examining variable “C,” I obviously need
to lock mutex “MutexABC.” What if I changed variable “E”? Well, before I change it,
I had to acquire the mutex “MutexDEF.” Then I changed it, and hit condvar
“CondvarABCDEF” to tell others about the change. Shortly thereafter, I would release
the mutex.
Now, consider what happens. Suddenly, I have a bunch of threads that had been
waiting on “CondvarABCDEF” that now wake up (from their pthread_cond_wait()).
The waiting function immediately attempts to reacquire the mutex. The critical point
here is that there are two mutexes to acquire. This means that on an SMP system, two
concurrent streams of threads can run, each examining what it considers to be
independent variables, using independent mutexes. Cool, eh?
Neutrino lets you do something else that’s elegant. POSIX says that a mutex must
operate between threads in the same process, and lets a conforming implementation
extend that. Neutrino extends this by allowing a mutex to operate between threads in
different processes. To understand why this works, recall that there really are two parts
to what’s viewed as the “operating system” — the kernel, which deals with scheduling,
68 Chapter 1 • Processes and Threads April 30, 2009