[m-rev.] for review: hang in lowlevel parallel grade

[Peter Wang]

Estimated hours taken: 6
Branches: main, release

This patch fixes a bug with lowlevel parallel grades.  A program built in such
a grade could hang when running with multiple threads.

runtime/mercury_context.c:
	After scheduling a Mercury context, use MR_BROADCAST to wake up all
	idle threads instead of MR_SIGNAL, if the newly scheduled context might
	not be accepted for execution by any single woken thread.  The hang
	used to occur when a context was scheduled but the wrong idle thread
	was woken up to execute it (because the context is 'owned' by another
	thread) and promptly went back to idling.

runtime/mercury_thread.h:
	Add MR_BROADCAST macros.


Index: runtime/mercury_context.c
===================================================================
RCS file: /home/mercury1/repository/mercury/runtime/mercury_context.c,v
retrieving revision 1.44
diff -u -r1.44 mercury_context.c
--- runtime/mercury_context.c	9 Mar 2006 08:24:41 -0000	1.44
+++ runtime/mercury_context.c	27 Jun 2006 05:48:21 -0000
@@ -347,7 +347,16 @@
         MR_runqueue_head = ctxt;
         MR_runqueue_tail = ctxt;
     }
-    MR_SIGNAL(&MR_runqueue_cond);
+    /*
+    ** Wake one or more threads waiting in MR_do_runnext.  If there is a
+    ** possibility that a woken thread might not accept this context then
+    ** we wake up all the waiting threads.
+    */
+    if (ctxt->MR_ctxt_owner_thread == (MercuryThread) NULL) {
+        MR_SIGNAL(&MR_runqueue_cond);
+    } else {
+        MR_BROADCAST(&MR_runqueue_cond);
+    }
     MR_UNLOCK(&MR_runqueue_lock, "schedule");
 }
 
Index: runtime/mercury_thread.h
===================================================================
RCS file: /home/mercury1/repository/mercury/runtime/mercury_thread.h,v
retrieving revision 1.17
diff -u -r1.17 mercury_thread.h
--- runtime/mercury_thread.h	20 Jun 2005 02:16:44 -0000	1.17
+++ runtime/mercury_thread.h	26 Jun 2006 10:50:35 -0000
@@ -46,6 +46,7 @@
     #define	MR_UNLOCK(lck, from)	pthread_mutex_unlock((lck))
 
     #define	MR_SIGNAL(cnd)		pthread_cond_signal((cnd))
+    #define	MR_BROADCAST(cnd)	pthread_cond_broadcast((cnd))
     #define	MR_WAIT(cnd, mtx)	pthread_cond_wait((cnd), (mtx))
   #else
     #define	MR_LOCK(lck, from)					\
@@ -67,6 +68,12 @@
 				:					\
 					pthread_cond_signal((cnd))	\
 				)
+    #define	MR_BROADCAST(cnd)					\
+    				( MR_debug_threads ?			\
+					MR_cond_broadcast((cnd))	\
+				:					\
+					pthread_cond_broadcast((cnd))	\
+				)
     #define	MR_WAIT(cnd, mtx)					\
     				( MR_debug_threads ?			\
 					MR_cond_wait((cnd), (mtx))	\
@@ -141,6 +148,7 @@
   #define MR_UNLOCK(nothing, from)	do { } while (0)
 
   #define MR_SIGNAL(nothing)		do { } while (0)
+  #define MR_BROADCAST(nothing)		do { } while (0)
   #define MR_WAIT(no, thing)		do { } while (0)
 
   #define MR_OBTAIN_GLOBAL_LOCK(where)	do { } while (0)
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