[hbelusca] 61239: [NTVDM] - Last race condition fix (see revision 61230). - Add a comment why we do twice the same emptiness check (one before holding the mutex and one just after).

Author: hbelusca
Date: Sat Dec  7 00:01:24 2013
New Revision: 61239

URL: http://svn.reactos.org/svn/reactos?rev=61239&view=rev
Log:
[NTVDM]
- Last race condition fix (see revision 61230).
- Add a comment why we do twice the same emptiness check (one before holding the mutex and
one just after).

Modified:
    branches/ntvdm/subsystems/ntvdm/ps2.c

Modified: branches/ntvdm/subsystems/ntvdm/ps2.c
URL:
http://svn.reactos.org/svn/reactos/branches/ntvdm/subsystems/ntvdm/ps2.c?re…
==============================================================================
--- branches/ntvdm/subsystems/ntvdm/ps2.c	[iso-8859-1] (original)
+++ branches/ntvdm/subsystems/ntvdm/ps2.c	[iso-8859-1] Sat Dec  7 00:01:24 2013
@@ -31,10 +31,16 @@
 
 static BOOLEAN KeyboardQueuePush(BYTE ScanCode)
 {
+    BOOLEAN Result = TRUE;
+
+    WaitForSingleObject(QueueMutex, INFINITE);
+
     /* Check if the keyboard queue is full */
-    WaitForSingleObject(QueueMutex, INFINITE);
-
-    if (!KeyboardQueueEmpty && (KeyboardQueueStart == KeyboardQueueEnd)) return
FALSE;
+    if (!KeyboardQueueEmpty && (KeyboardQueueStart == KeyboardQueueEnd))
+    {
+        Result = FALSE;
+        goto Done;
+    }
 
     /* Insert the value in the queue */
     KeyboardQueue[KeyboardQueueEnd] = ScanCode;
@@ -44,19 +50,24 @@
     /* Since we inserted a value, it's not empty anymore */
     KeyboardQueueEmpty = FALSE;
 
+Done:
     ReleaseMutex(QueueMutex);
-    return TRUE;
+    return Result;
 }
 
 static BOOLEAN KeyboardQueuePop(BYTE *ScanCode)
 {
     BOOLEAN Result = TRUE;
 
-    /* Make sure the keyboard queue is not empty */
+    /* Make sure the keyboard queue is not empty (fast check) */
     if (KeyboardQueueEmpty) return FALSE;
 
     WaitForSingleObject(QueueMutex, INFINITE);
 
+    /*
+     * Recheck whether keyboard queue is not empty (it may
+     * have been changed after having grabbed the mutex).
+     */
     if (KeyboardQueueEmpty)
     {
         Result = FALSE;