From 6e95a132bd80c0019df6cbfa4494dea619e8a4d3 Mon Sep 17 00:00:00 2001
From: Ahmed Arif <arif.ing@outlook.com>
Date: Fri, 26 Jun 2026 14:07:15 +0200
Subject: [PATCH] [WIN32SS:ENG/NTUSER] Use shared locks for display list
 lookups (#9090)

- Use shared locks for the display device and PDEV lookup paths.
  These functions only walk existing lists and take references, while list updates
  and mode-switch paths still use exclusive locking.
  This avoids serializing read-only display queries unnecessarily.

- Use shared locks for other read-only NtUser calls.
---
 win32ss/gdi/eng/device.c         |  2 +-
 win32ss/gdi/eng/pdevobj.c        |  2 +-
 win32ss/user/ntuser/menu.c       |  2 +-
 win32ss/user/ntuser/simplecall.c | 32 +++++++++++++++++++++++++++++---
 win32ss/user/ntuser/window.c     |  2 +-
 win32ss/user/ntuser/winpos.c     |  2 +-
 6 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/win32ss/gdi/eng/device.c b/win32ss/gdi/eng/device.c
index 384d4c960c8..81592b88f41 100644
--- a/win32ss/gdi/eng/device.c
+++ b/win32ss/gdi/eng/device.c
@@ -803,7 +803,7 @@ EngpFindGraphicsDevice(
            pustrDevice, iDevNum);
 
     /* Lock list */
-    EngAcquireSemaphore(ghsemGraphicsDeviceList);
+    EngAcquireSemaphoreShared(ghsemGraphicsDeviceList);
 
     if (pustrDevice && pustrDevice->Buffer)
     {
diff --git a/win32ss/gdi/eng/pdevobj.c b/win32ss/gdi/eng/pdevobj.c
index 9bc2f88684e..90c81ed59a9 100644
--- a/win32ss/gdi/eng/pdevobj.c
+++ b/win32ss/gdi/eng/pdevobj.c
@@ -805,7 +805,7 @@ EngpGetPDEV(
     ULONG i;
 
     /* Acquire PDEV lock */
-    EngAcquireSemaphore(ghsemPDEV);
+    EngAcquireSemaphoreShared(ghsemPDEV);
 
     /* Did the caller pass a device name? */
     if (pustrDeviceName)
diff --git a/win32ss/user/ntuser/menu.c b/win32ss/user/ntuser/menu.c
index 4d14fe1b55d..e4ade4016e6 100644
--- a/win32ss/user/ntuser/menu.c
+++ b/win32ss/user/ntuser/menu.c
@@ -6315,7 +6315,7 @@ NtUserMenuItemFromPoint(
    int Ret = -1;
 
    TRACE("Enter NtUserMenuItemFromPoint\n");
-   UserEnterExclusive();
+   UserEnterShared();
 
    if (!(Menu = UserGetMenuObject(hMenu)))
    {
diff --git a/win32ss/user/ntuser/simplecall.c b/win32ss/user/ntuser/simplecall.c
index 6a433ed7120..412b9fea2b1 100644
--- a/win32ss/user/ntuser/simplecall.c
+++ b/win32ss/user/ntuser/simplecall.c
@@ -61,7 +61,19 @@ NtUserCallNoParam(DWORD Routine)
     DWORD_PTR Result = 0;
 
     TRACE("Enter NtUserCallNoParam\n");
-    UserEnterExclusive();
+
+    /* Read-only routines only need a shared lock; the rest stay exclusive */
+    switch (Routine)
+    {
+        case NOPARAM_ROUTINE_GETMSESSAGEPOS:
+        case NOPARAM_ROUTINE_GETIMESHOWSTATUS:
+        case NOPARAM_ROUTINE_ISCONSOLEMODE:
+            UserEnterShared();
+            break;
+        default:
+            UserEnterExclusive();
+            break;
+    }
 
     switch (Routine)
     {
@@ -158,7 +170,22 @@ NtUserCallOneParam(
 
     TRACE("Enter NtUserCallOneParam\n");
 
-    UserEnterExclusive();
+    /* Read-only routines only need a shared lock; the rest stay exclusive */
+    switch (Routine)
+    {
+        case ONEPARAM_ROUTINE_GETDESKTOPMAPPING:
+        case ONEPARAM_ROUTINE_WINDOWFROMDC:
+        case ONEPARAM_ROUTINE_GETKEYBOARDTYPE:
+        case ONEPARAM_ROUTINE_GETKEYBOARDLAYOUT:
+        case ONEPARAM_ROUTINE_ENUMCLIPBOARDFORMATS:
+        case ONEPARAM_ROUTINE_GETCURSORPOS:
+        case ONEPARAM_ROUTINE_GETPROCDEFLAYOUT:
+            UserEnterShared();
+            break;
+        default:
+            UserEnterExclusive();
+            break;
+    }
 
     switch (Routine)
     {
@@ -319,7 +346,6 @@ NtUserCallOneParam(
         }
 
         case ONEPARAM_ROUTINE_ENUMCLIPBOARDFORMATS:
-            /* FIXME: Should use UserEnterShared */
             Result = UserEnumClipboardFormats(Param);
             break;
 
diff --git a/win32ss/user/ntuser/window.c b/win32ss/user/ntuser/window.c
index 49ee4d3bcb0..58bf7c7c2d3 100644
--- a/win32ss/user/ntuser/window.c
+++ b/win32ss/user/ntuser/window.c
@@ -1527,7 +1527,7 @@ NtUserBuildHwndList(
    if (pcHwndNeeded == NULL)
        return STATUS_INVALID_PARAMETER;
 
-   UserEnterExclusive();
+   UserEnterShared();
 
    if (hwndParent || !dwThreadId)
    {
diff --git a/win32ss/user/ntuser/winpos.c b/win32ss/user/ntuser/winpos.c
index 43a52ab5cf8..74c0563e8a4 100644
--- a/win32ss/user/ntuser/winpos.c
+++ b/win32ss/user/ntuser/winpos.c
@@ -3283,7 +3283,7 @@ NtUserChildWindowFromPointEx(HWND hwndParent,
 {
    PWND pwndParent;
    TRACE("Enter NtUserChildWindowFromPointEx\n");
-   UserEnterExclusive();
+   UserEnterShared();
    if ((pwndParent = UserGetWindowObject(hwndParent)))
    {
       pwndParent = IntChildWindowFromPointEx(pwndParent, x, y, uiFlags);