[PATCH] x86:pvclock: add missing barriers

[Roman Kagan <rkagan@virtuozzo.com>]

Gradual removal of excessive barriers in pvclock reading functions
(commits 502dfeff239e8313bfbe906ca0a1a6827ac8481b,
a3eb97bd80134ba07864ca00747466c02118aca1) ended up removing too much:
although rdtsc is now orderd WRT other loads, there's no protection
against the compiler reordering the loads of ->version with the loads of
other fields.

E.g. on my system gcc-5.3.1 generates code which loads ->system_time and
->flags outside of the ->version test loop.

(Re)introduce the compiler barriers around accesses to the contents of
pvclock.  While at this, make the function a bit more compact by
removing unnecessary local variables.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
---
 arch/x86/include/asm/pvclock.h | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index fdcc040..65c4de2 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -80,18 +80,11 @@ static __always_inline
 unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
 			       cycle_t *cycles, u8 *flags)
 {
-	unsigned version;
-	cycle_t ret, offset;
-	u8 ret_flags;
-
-	version = src->version;
-
-	offset = pvclock_get_nsec_offset(src);
-	ret = src->system_time + offset;
-	ret_flags = src->flags;
-
-	*cycles = ret;
-	*flags = ret_flags;
+	unsigned version = src->version;
+	barrier();
+	*cycles = src->system_time + pvclock_get_nsec_offset(src);
+	*flags = src->flags;
+	barrier();
 	return version;
 }
 
-- 
2.5.5