kernel/smp: Add a comment for some obsecure knowledge

* I was ready to rip this out until PulkoMandy set me stright.
* Add a comment so others understand the impact here.

headers/private/kernel/arch/ppc/arch_atomic.h

@@ -1,110 +0,0 @@
-/*
- * Copyright 2014, Paweł Dziepak, [email protected].
- * Distributed under the terms of the MIT License.
- *
- * Authors:
- *		Alexander von Gluck IV <[email protected]>
- */
-#ifndef _KERNEL_ARCH_PPC_ATOMIC_H
-#define _KERNEL_ARCH_PPC_ATOMIC_H
-
-
-static inline void
-memory_read_barrier_inline(void)
-{
-	#ifdef __powerpc64__
-	asm volatile("lwsync" : : : "memory");
-	#else
-	asm volatile("sync" : : : "memory");
-	#endif
-}
-
-
-static inline void
-memory_write_barrier_inline(void)
-{
-	#ifdef __powerpc64__
-	asm volatile("lwsync" : : : "memory");
-	#else
-	asm volatile("eieio" : : : "memory");
-	#endif
-}
-
-
-static inline void
-memory_full_barrier_inline(void)
-{
-	asm volatile("sync" : : : "memory");
-}
-
-
-#define memory_read_barrier		memory_read_barrier_inline
-#define memory_write_barrier	memory_write_barrier_inline
-#define memory_full_barrier		memory_full_barrier_inline
-
-
-static inline void
-atomic_set_inline(int32* value, int32 newValue)
-{
-	memory_write_barrier();
-	*(volatile int32*)value = newValue;
-}
-
-
-static inline int32
-atomic_get_and_set_inline(int32* value, int32 newValue)
-{
-	// BIG TODO: PowerPC Atomic get and set
-//	asm volatile("xchgl %0, (%1)"
-//		: "+r" (newValue)
-//		: "r" (value)
-//		: "memory");
-	return newValue;
-}
-
-
-static inline int32
-atomic_test_and_set_inline(int32* value, int32 newValue, int32 testAgainst)
-{
-	// BIG TODO: PowerPC Atomic test and set inline
-//	asm volatile("lock; cmpxchgl %2, (%3)"
-//		: "=a" (newValue)
-//		: "0" (testAgainst), "r" (newValue), "r" (value)
-//		: "memory");
-	return newValue;
-}
-
-
-static inline int32
-atomic_add_inline(int32* value, int32 newValue)
-{
-	// BIG TODO: PowerPC Atomic add inline
-//	asm volatile("lock; xaddl %0, (%1)"
-//		: "+r" (newValue)
-//		: "r" (value)
-//		: "memory");
-	return newValue;
-}
-
-
-static inline int32
-atomic_get_inline(int32* value)
-{
-	int32 newValue = *(volatile int32*)value;
-	memory_read_barrier();
-	return newValue;
-}
-
-
-#define atomic_set				atomic_set_inline
-#define atomic_get_and_set		atomic_get_and_set_inline
-#ifndef atomic_test_and_set
-#	define atomic_test_and_set	atomic_test_and_set_inline
-#endif
-#ifndef atomic_add
-#	define atomic_add			atomic_add_inline
-#endif
-#define atomic_get				atomic_get_inline
-
-
-#endif	// _KERNEL_ARCH_PPC_ATOMIC_H

src/system/kernel/smp.cpp

@@ -1540,6 +1540,9 @@ call_all_cpus_sync(void (*func)(void*, int), void* cookie)
 }
 
 
+// Ensure the symbols for memory_barriers are still included
+// in the kernel for binary compatibility. Calls are forwarded
+// to the more efficent per-processor atomic implementations.
 #undef memory_read_barrier
 #undef memory_write_barrier