powerpc/kcsan: Properly instrument arch_spin_unlock()

The following boottime error is encountered with SMP kernel:

kcsan: improperly instrumented type=(0): arch_spin_unlock(&arch_spinlock)
kcsan: improperly instrumented type=(0): spin_unlock(&test_spinlock)
kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE): arch_spin_unlock(&arch_spinlock)
kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE): spin_unlock(&test_spinlock)
kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND): arch_spin_unlock(&arch_spinlock)
kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND): spin_unlock(&test_spinlock)
kcsan: selftest: test_barrier failed
kcsan: selftest: 2/3 tests passed
Kernel panic - not syncing: selftests failed

Properly instrument arch_spin_unlock() with kcsan_mb().

Signed-off-by: Christophe Leroy <[e-mail]>
Acked-by: Marco Elver <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/57834a70[e-mail]

arch/powerpc/include/asm/simple_spinlock.h

···
  * (the type definitions are in asm/simple_spinlock_types.h)
  */
 #include <linux/irqflags.h>
+#include <linux/kcsan-checks.h>
 #include <asm/paravirt.h>
 #include <asm/paca.h>
 #include <asm/synch.h>
···
 
 static inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
+	kcsan_mb();
 	__asm__ __volatile__("# arch_spin_unlock\n\t"
 				PPC_RELEASE_BARRIER: : :"memory");
 	lock->slock = 0;