diff -r 4a8fed1c0ef6 -r 597aaf25e343 kernel/eka/memmodel/epoc/flexible/mmu/arm/xmmu.cpp
--- a/kernel/eka/memmodel/epoc/flexible/mmu/arm/xmmu.cpp	Sat Feb 20 00:10:51 2010 +0200
+++ b/kernel/eka/memmodel/epoc/flexible/mmu/arm/xmmu.cpp	Fri Mar 12 15:50:11 2010 +0200
@@ -928,13 +928,11 @@
 	// Now we have the os asid check access to kernel memory.
 	if(aAddr >= KUserMemoryLimit && osAsid != (TUint)KKernelOsAsid)
 		{
-		NKern::ThreadEnterCS();
-		MmuLock::Unlock();
 		if (!iAliasLinAddr)
 			{// Close the new reference as RemoveAlias won't do as iAliasLinAddr is not set.
 			aProcess->AsyncCloseOsAsid();	// Asynchronous close as this method should be quick.
 			}
-		NKern::ThreadLeaveCS();
+		MmuLock::Unlock();
 		return KErrBadDescriptor; // prevent access to supervisor only memory
 		}
 
@@ -944,10 +942,8 @@
 		// address is in global section, don't bother aliasing it...
 		if (!iAliasLinAddr)
 			{// Close the new reference as not required.
-			NKern::ThreadEnterCS();
+			aProcess->AsyncCloseOsAsid(); // Asynchronous close as this method should be quick.
 			MmuLock::Unlock();
-			aProcess->AsyncCloseOsAsid(); // Asynchronous close as this method should be quick.
-			NKern::ThreadLeaveCS();
 			}
 		else
 			{// Remove the existing alias as it is not required.
@@ -1061,15 +1057,9 @@
 	iCpuRestoreCookie = -1;
 #endif
 
-	// Must close the os asid while in critical section to prevent it being 
-	// leaked.  However, we can't hold the mmu lock so we have to enter an 
-	// explict crtical section. It is ok to release the mmu lock as the 
-	// iAliasLinAddr and iAliasProcess members are only ever updated by the 
-	// current thread.
-	NKern::ThreadEnterCS();
+	// Must close the os asid while holding MmuLock so we are in an implicit critical section.
+	iAliasProcess->AsyncCloseOsAsid(); // Asynchronous close as this method should be quick.
 	MmuLock::Unlock();
-	iAliasProcess->AsyncCloseOsAsid(); // Asynchronous close as this method should be quick.
-	NKern::ThreadLeaveCS();
 	}