RFC - multithreaded exceptions support for single CPU embedded systems

[Øyvind Harboe <oyvind dot harboe at zylin dot com>]

I'm not terribly familiar with exceptions, but I've made some
very simple modifications to eCos and GCC code to add support
for multithreaded exceptions. I needed this for development
purposes and it seems to work quite well.

Q: is this a sound method for adding multithreaded support to
deeply embedded targets?

Øyvind

Index: unwind-sjlj.c
===================================================================
RCS file: /cvsroot/gcc/gcc/gcc/unwind-sjlj.c,v
retrieving revision 1.14
diff -u -r1.14 unwind-sjlj.c
--- unwind-sjlj.c	7 May 2003 22:11:34 -0000	1.14
+++ unwind-sjlj.c	16 Jul 2003 08:26:20 -0000
@@ -86,8 +86,25 @@
 
 /* Manage the chain of registered function contexts.  */
 
-/* Single threaded fallback chain.  */
-static struct SjLj_Function_Context *fc_static;
+/* Single threaded(or multithreaded single cpu) fallback 
+ * chain.
+ * 
+ * Notice that this is a globally visible variable.
+ * 
+ * The way the single threaded callback chain is extended to a
multithreaded
+ * callback chain, is that the scheduler during a thread switch saves 
+ * the value of this variable in the thread state/structure for the
thread
+ * being switched out, and it restores the value from the thread being
+ * switched in.
+ * 
+ * Thread startup routines are modified to initialize this value per
thread. 
+ * 
+ * This scheme only works for single CPU systems, but it is an
efficient 
+ * and simple method method to add multithreaded
+ * exceptions to e.g. sources.redhat.com/eCos.
+ * 
+ */
+struct SjLj_Function_Context *fc_static;
 
 #if __GTHREADS
 static __gthread_key_t fc_key;