Implement new 'lightweight lock manager' that's intermediate between

existing lock manager and spinlocks: it understands exclusive vs shared
lock but has few other fancy features.  Replace most uses of spinlocks
with lightweight locks.  All remaining uses of spinlocks have very short
lock hold times (a few dozen instructions), so tweak spinlock backoff
code to work efficiently given this assumption.  All per my proposal on
pghackers 26-Sep-01.

src/backend/storage/ipc/sinvaladt.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.40 2001/06/19 19:42:15 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.41 2001/09/29 04:02:24 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -83,7 +83,7 @@ SIBufferInit(int maxBackends)
  *	   <0	Some other failure (not currently used)
  *
  * NB: this routine, and all following ones, must be executed with the
- * SInvalLock spinlock held, since there may be multiple backends trying
+ * SInvalLock lock held, since there may be multiple backends trying
  * to access the buffer.
  */
 int
@@ -152,7 +152,7 @@ CleanupInvalidationState(int status, Datum arg)
 
 	Assert(PointerIsValid(segP));
 
-	SpinAcquire(SInvalLock);
+	LWLockAcquire(SInvalLock, LW_EXCLUSIVE);
 
 	/* Mark myself inactive */
 	segP->procState[MyBackendId - 1].nextMsgNum = -1;
@@ -167,7 +167,7 @@ CleanupInvalidationState(int status, Datum arg)
 	}
 	segP->lastBackend = i;
 
-	SpinRelease(SInvalLock);
+	LWLockRelease(SInvalLock);
 }
 
 /*
@@ -267,6 +267,10 @@ SISetProcStateInvalid(SISeg *segP)
  *	1: next SI message has been extracted into *data
  *		(there may be more messages available after this one!)
  * -1: SI reset message extracted
+ *
+ * NB: this can run in parallel with other instances of SIGetDataEntry
+ * executing on behalf of other backends.  See comments in sinval.c in
+ * ReceiveSharedInvalidMessages().
  */
 int
 SIGetDataEntry(SISeg *segP, int backendId,