x86/time: fix wc_version retry check

When using | instead of || (attempting to make the compiler issue just
a single branch), both sides of the operator aren't separated by a
sequence point, and hence evaluation can happen in any order. In
particular the rightmost read of s->wc_version could get carried out
before the leftmost one. Use a local variable to prevent this. (In
reality the compiler is very likely to do only a single memory read
here anyway.)

Reported-by: Julian Stecklina <jsteckli@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/arch/i386/kernel/time-xen.c
+++ b/arch/i386/kernel/time-xen.c
@@ -257,6 +257,7 @@
 static void update_wallclock(void)
 {
 	shared_info_t *s = HYPERVISOR_shared_info;
+	u32 version;
 
 	do {
 		shadow_tv_version = s->wc_version;
@@ -264,7 +265,8 @@
 		shadow_tv.tv_sec  = s->wc_sec;
 		shadow_tv.tv_nsec = s->wc_nsec;
 		rmb();
-	} while ((s->wc_version & 1) | (shadow_tv_version ^ s->wc_version));
+		version = s->wc_version;
+	} while ((version & 1) | (shadow_tv_version ^ version));
 
 	if (!independent_wallclock)
 		__update_wallclock(shadow_tv.tv_sec, shadow_tv.tv_nsec);