Pvt deadlock causes some channels to get stuck in Reserved status.

(closes issue #12621)
 Reported by: fabianoheringer
 Patches: 
       20080612__bug12621.diff.txt uploaded by Corydon76 (license 14)
 Tested by: fabianoheringer


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@124965 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_dahdi.c

@@ -4637,10 +4637,11 @@ static struct ast_frame  *dahdi_read(struct ast_channel *ast)
 	int index;
 	void *readbuf;
 	struct ast_frame *f;
-	
 
-	ast_mutex_lock(&p->lock);
-	
+	while (ast_mutex_trylock(&p->lock)) {
+		DEADLOCK_AVOIDANCE(&ast->lock);
+	}
+
 	index = dahdi_get_index(ast, p, 0);
 	
 	/* Hang up if we don't really exist */