[POWERPC] Make mmiowb's io_sync preempt safe

If mmiowb() is always used prior to releasing spinlock as Doc suggests,
then it's safe against preemption; but I'm not convinced that's always
the case.  If preemption occurs between sync and get_paca()->io_sync = 0,
I believe there's no problem.  But in the unlikely event that gcc does
the store relative to another register than r13 (as it did with current),
then there's a small danger of setting another cpu's io_sync to 0, after
it had just set it to 1.  Rewrite ppc64 mmiowb to prevent that.

The remaining io_sync assignments in io.h all get_paca()->io_sync = 1,
which is harmless even if preempted to the wrong cpu (the context switch
itself syncs); and those in spinlock.h are while preemption is disabled.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/io.h

@@ -163,8 +163,11 @@ extern void _outsl_ns(volatile u32 __iomem *port, const void *buf, long count);
 
 static inline void mmiowb(void)
 {
-	__asm__ __volatile__ ("sync" : : : "memory");
-	get_paca()->io_sync = 0;
+	unsigned long tmp;
+
+	__asm__ __volatile__("sync; li %0,0; stb %0,%1(13)"
+	: "=&r" (tmp) : "i" (offsetof(struct paca_struct, io_sync))
+	: "memory");
 }
 
 /*