hw: unlock kernel before context propagation

Propagating the user context-pointer from C++ code to the mode
transition assembly doesn't touch any CPU global data. Thus, we can
reduce the in-sync window.

Fixes #1223

repos/base-hw/src/core/kernel/pd.cc

@@ -69,14 +69,14 @@ void Mode_transition_control::switch_to(Cpu::Context * const context,
                                         addr_t const entry_raw,
                                         addr_t const context_ptr_base)
 {
+	/* unlock kernel data */
+	data_lock().unlock();
+
 	/* override client-context pointer of the executing CPU */
 	size_t const context_ptr_offset = cpu * sizeof(context);
 	addr_t const context_ptr = context_ptr_base + context_ptr_offset;
 	*(void * *)context_ptr = context;
 
-	/* unlock kernel data */
-	data_lock().unlock();
-
 	/* call assembly code that applies the virtual-machine context */
 	typedef void (* Entry)();
 	Entry __attribute__((noreturn)) const entry = (Entry)entry_raw;