diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index c0deafc212b8..25d42bd3f114 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h
@@ -74,6 +74,11 @@ do {									\
 	___p1;								\
 })
 
+/*
+ * This must resolve to hwsync on SMP for the context switch path.
+ * See _switch, and core scheduler context switch memory ordering
+ * comments.
+ */
 #define smp_mb__before_spinlock()   smp_mb()
 
 #include <asm-generic/barrier.h>
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 273a35926534..fb143859cc68 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -512,13 +512,24 @@ _GLOBAL(_switch)
 	std	r23,_CCR(r1)
 	std	r1,KSP(r3)	/* Set old stack pointer */
 
-#ifdef CONFIG_SMP
-	/* We need a sync somewhere here to make sure that if the
-	 * previous task gets rescheduled on another CPU, it sees all
-	 * stores it has performed on this one.
+	/*
+	 * On SMP kernels, care must be taken because a task may be
+	 * scheduled off CPUx and on to CPUy. Memory ordering must be
+	 * considered.
+	 *
+	 * Cacheable stores on CPUx will be visible when the task is
+	 * scheduled on CPUy by virtue of the core scheduler barriers
+	 * (see "Notes on Program-Order guarantees on SMP systems." in
+	 * kernel/sched/core.c).
+	 *
+	 * Uncacheable stores in the case of involuntary preemption must
+	 * be taken care of. The smp_mb__before_spin_lock() in __schedule()
+	 * is implemented as hwsync on powerpc, which orders MMIO too. So
+	 * long as there is an hwsync in the context switch path, it will
+	 * be executed on the source CPU after the task has performed
+	 * all MMIO ops on that CPU, and on the destination CPU before the
+	 * task performs any MMIO ops there.
 	 */
-	sync
-#endif /* CONFIG_SMP */
 
 	/*
 	 * The kernel context switch path must contain a spin_lock,