powerpc: Implement smp_cond_load_relaxed()

This implements smp_cond_load_relaxed() with the slowpath busy loop
using the preferred SMT priority pattern.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Waiman Long <longman@redhat.com>
[mpe: Make it 64-bit only to fix build errors on 32-bit]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131423.1362108-7-npiggin@gmail.com

arch/powerpc/include/asm/barrier.h

@@ -80,6 +80,22 @@ do {									\
 	___p1;								\
 })
 
+#ifdef CONFIG_PPC64
+#define smp_cond_load_relaxed(ptr, cond_expr) ({		\
+	typeof(ptr) __PTR = (ptr);				\
+	__unqual_scalar_typeof(*ptr) VAL;			\
+	VAL = READ_ONCE(*__PTR);				\
+	if (unlikely(!(cond_expr))) {				\
+		spin_begin();					\
+		do {						\
+			VAL = READ_ONCE(*__PTR);		\
+		} while (!(cond_expr));				\
+		spin_end();					\
+	}							\
+	(typeof(*ptr))VAL;					\
+})
+#endif
+
 #ifdef CONFIG_PPC_BOOK3S_64
 #define NOSPEC_BARRIER_SLOT   nop
 #elif defined(CONFIG_PPC_FSL_BOOK3E)