From: Chris Hanson <org/chris-hanson/cph>
Date: Sat, 22 Apr 1995 21:17:54 +0000 (+0000)
Subject: Lock access to the twin of a qid more carefully.  Previously, the code
X-Git-Tag: 20090517-FFI~6420
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=d08192bd4a74fdc5f65fa0a1c3a43c441ec231af;p=mit-scheme.git

Lock access to the twin of a qid more carefully.  Previously, the code
for sending messages was not locked at all, which meant that there
were race conditions that could produce fatal errors.  By locking, we
limit the amount of concurrent communication slightly, but since the
locks are active for only a short time this shouldn't be noticed.

Also implement a new procedure OS2_qid_tqueue, which is used by
inferior threads to obtain and close their associated tqueue objects
when they finish.
---

diff --git a/v7/src/microcode/os2msg.c b/v7/src/microcode/os2msg.c
index 6cf4b4306..f94c826c6 100644
--- a/v7/src/microcode/os2msg.c
+++ b/v7/src/microcode/os2msg.c
@@ -1,6 +1,6 @@
 /* -*-C-*-
 
-$Id: os2msg.c,v 1.6 1995/04/11 05:19:34 cph Exp $
+$Id: os2msg.c,v 1.7 1995/04/22 21:17:36 cph Exp $
 
 Copyright (c) 1994-95 Massachusetts Institute of Technology
 
@@ -165,7 +165,6 @@ OS2_close_qid (qid_t qid)
       OS2_destroy_message (this -> message);
       OS_free (this);
     }
-  (QID_TQUEUE (qid)) = 0;
   OS2_request_mutex_semaphore (qid_lock);
   {
     qid_t twin = (QID_TWIN (qid));
@@ -175,27 +174,48 @@ OS2_close_qid (qid_t qid)
 	(QID_TWIN (qid)) = QID_NONE;
       }
   }
+  (QID_TQUEUE (qid)) = 0;
   (QID_ALLOCATEDP (qid)) = 0;
   OS2_release_mutex_semaphore (qid_lock);
 }
 
+tqueue_t *
+OS2_qid_tqueue (qid_t qid)
+{
+  return (QID_TQUEUE (qid));
+}
+
 qid_t
 OS2_qid_twin (qid_t qid)
 {
-  return (((QID_ALLOCATEDP (qid))
-	   && ((QID_TWIN (qid)) != QID_NONE)
-	   && (QID_ALLOCATEDP (QID_TWIN (qid))))
-	  ? (QID_TWIN (qid))
-	  : QID_NONE);
+  qid_t twin;
+  OS2_request_mutex_semaphore (qid_lock);
+  twin
+    = (((QID_ALLOCATEDP (qid))
+	&& ((QID_TWIN (qid)) != QID_NONE)
+	&& (QID_ALLOCATEDP (QID_TWIN (qid))))
+       ? (QID_TWIN (qid))
+       : QID_NONE);
+  OS2_release_mutex_semaphore (qid_lock);
+  return (twin);
 }
 
 void
 OS2_close_qid_pair (qid_t qid)
 {
+  /* This is safe because it is used only in a particular way.  The
+     twin of this qid is never received from, and qid is never sent
+     to, and the twin will never be closed by the other thread.  Thus,
+     even though the unlocked sections of OS2_close_qid are
+     manipulating structures that belong to the other thread, the
+     other thread won't be manipulating them so no conflict will
+     arise.  It's important not to use this procedure in any other
+     situation!  */
   if (QID_ALLOCATEDP (qid))
     {
-      if (((QID_TWIN (qid)) != QID_NONE) && (QID_ALLOCATEDP (QID_TWIN (qid))))
-	OS2_close_qid (QID_TWIN (qid));
+      qid_t twin = (OS2_qid_twin (qid));
+      if (twin != QID_NONE)
+	OS2_close_qid (twin);
       OS2_close_qid (qid);
     }
 }
@@ -297,7 +317,9 @@ OS2_destroy_message (msg_t * message)
 void
 OS2_send_message (qid_t qid, msg_t * message)
 {
-  qid_t twin = (QID_TWIN (qid));
+  qid_t twin;
+  OS2_request_mutex_semaphore (qid_lock);
+  twin = (QID_TWIN (qid));
   if (twin == QID_NONE)
     /* Other end of connection has been closed, so discard the
        message.  We used to signal an error here, but this can happen
@@ -317,6 +339,7 @@ OS2_send_message (qid_t qid, msg_t * message)
       (MSG_SENDER (message)) = twin;
       write_tqueue (tqueue, message);
     }
+  OS2_release_mutex_semaphore (qid_lock);
 }
 
 msg_t *
diff --git a/v7/src/microcode/os2msg.h b/v7/src/microcode/os2msg.h
index b653a74dc..2c65e3d3c 100644
--- a/v7/src/microcode/os2msg.h
+++ b/v7/src/microcode/os2msg.h
@@ -1,6 +1,6 @@
 /* -*-C-*-
 
-$Id: os2msg.h,v 1.7 1995/02/21 22:54:30 cph Exp $
+$Id: os2msg.h,v 1.8 1995/04/22 21:17:54 cph Exp $
 
 Copyright (c) 1994-95 Massachusetts Institute of Technology
 
@@ -210,6 +210,7 @@ extern void OS2_make_qid_pair (qid_t *, qid_t *);
 extern void OS2_open_qid (qid_t, tqueue_t *);
 extern int OS2_qid_openp (qid_t);
 extern void OS2_close_qid (qid_t);
+extern tqueue_t * OS2_qid_tqueue (qid_t);
 extern qid_t OS2_qid_twin (qid_t);
 extern void OS2_close_qid_pair (qid_t);
 extern void OS2_set_qid_receive_filter (qid_t, qid_receive_filter_t);