[tip:x86/urgent] x86/vdso/pvclock: Protect STABLE check with the seqcount

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  78fd8c7288e0a4bba3ad1d69caf9396a6b69cb00
Gitweb:     http://git.kernel.org/tip/78fd8c7288e0a4bba3ad1d69caf9396a6b69cb00
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Mon, 4 Jan 2016 15:14:28 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 13 Jan 2016 11:46:29 +0100

x86/vdso/pvclock: Protect STABLE check with the seqcount

If the clock becomes unstable while we're reading it, we need to
bail.  We can do this by simply moving the check into the
seqcount loop.

Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Alexander Graf <agraf@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krcmar <rkrcmar@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/755dcedb17269e1d7ce12a9a713dea303835137e.1451949191.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/entry/vdso/vclock_gettime.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 8602f06..1a50e09 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -126,23 +126,23 @@ static notrace cycle_t vread_pvclock(int *mode)
 	 *
 	 * On Xen, we don't appear to have that guarantee, but Xen still
 	 * supplies a valid seqlock using the version field.
-
+	 *
 	 * We only do pvclock vdso timing at all if
 	 * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
 	 * mean that all vCPUs have matching pvti and that the TSC is
 	 * synced, so we can just look at vCPU 0's pvti.
 	 */
 
-	if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
-		*mode = VCLOCK_NONE;
-		return 0;
-	}
-
 	do {
 		version = pvti->version;
 
 		smp_rmb();
 
+		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
+			*mode = VCLOCK_NONE;
+			return 0;
+		}
+
 		tsc = rdtsc_ordered();
 		pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
 		pvti_tsc_shift = pvti->tsc_shift;