gstreamer/docs/random/thomasvs/pthread

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Some notes on use of pthreads in GStreamer
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First off, pthreads are HARD. Remember that.
1) How I learned to debug glibc and pthreads and add debug code to it.
You have to trick your GStreamer test app in running against a modified
glibc.
I used Red Hat 7.3, downloaded the .src.rpm, installed it, applied the
patches included, and ran
./configure --prefix=/home/thomas/cvs --with-add-ons
make
make install
After quite some time this left me with recompiled libc and libpthread
libraries in /home/thomas/cvs/lib, as well as a new ld-linux.so.2
Now you need to use this new ld-linux.so ld loader to run your app,
preferably from inside of gdb so you can tell what's going on when it
crashes.
You can use ld-linux.so.2 to call your binaries:
ld-linux.so.2 .libs/thread1
to run the thread1 program with the new glibc.
If this is a GStreamer app, chances are it might not find some libraries
it needs that you could safely use from /usr/lib (like, zlib and popt).
Also, you want it to run in gdb, so this is my full line:
LD_LIBRARY_PATH=/usr/lib /home/thomas/cvs/lib/ld-linux.so.2 \
/usr/bin/gdb .libs/thread1
At this point you can start adding debug code to the pthreads implementation
in your glibc source tree. Just change, re-run make install, and restart
the test app in gdb.
Helpful --gst-mask is 0x00200100 to get thread info and scheduling info
(with mem alloc from cothreads)
2) What GStreamer does with pthreads.
Apps create a thread with gst_thread_new. This just allocates the GstThread
structure without actually doing much with it.
When a thread goes from NULL to READY, the gst_thread_change_state function
creates the actual pthread.
- we lock the thread->lock mutex
- we create attributes for the pthread
- by default the pthread is JOINABLE
- we ask the thread's scheduler for a preferred stack size and location
(FIXME: if the scheduler doesn't return one, what do we do ?)
- we create the pthread with the given attributes
- the pthread id is stored in thread->thread_id
- the created pthread starts executing gst_thread_main_loop (thread)
- the change_state function does a g_cond_wait
- this means it unlocks the mutex, waits until thread->cond is set
(which happens in gst_thread_main_loop),
then relocks the mutex and resumes execution
From the point of view of the created pthread, here's what happens.
gst_thread_main_loop (thread) gets called
- the thread's mutex gets locked
- the thread's scheduler's policy gets examined
- the scheduler gets set up (invokes the scheduler object's setup method)
FIXME: what are the prereqs of this _setup method ?
- basic and fast scheduler both call do_cothread_context_init
- basic: this calls cothread_context_init
- cothread_context_init
- fast: this calls cothread_create (NULL, 0, NULL, NULL))
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