linux-kernel - Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read

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Message-ID: <20160609122404.GB30927@twins.programming.kicks-ass.net>
Date:	Thu, 9 Jun 2016 14:24:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Minfei Huang <mnghuan@...il.com>
Cc:	bp@...e.de, pbonzini@...hat.com, luto@...nel.org, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in
 function pvclock_read_flags

On Thu, Jun 09, 2016 at 02:16:03PM +0200, Peter Zijlstra wrote:
> What's wrong with:
> 
> 	u8 flags = READ_ONCE(src->flags);
> 
> ?
> 
> (and have the flags store be done using WRITE_ONCE() of course).
> 
> Sure, if your total state is larger than one word you need the
> seqcount for integrity, but reading _one_ byte, shees.

Something like so; although given that I've never seen this code before
I can easily have missed an update site.

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 99bfc025111d..7a4bf59aa929 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -60,15 +60,7 @@ void pvclock_resume(void)
 
 u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
 {
-	unsigned version;
-	cycle_t ret;
-	u8 flags;
-
-	do {
-		version = __pvclock_read_cycles(src, &ret, &flags);
-	} while ((src->version & 1) || version != src->version);
-
-	return flags & valid_flags;
+	return READ_ONCE(src->flags) & valid_flags;
 }
 
 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
@@ -83,7 +75,8 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 	} while ((src->version & 1) || version != src->version);
 
 	if (unlikely((flags & PVCLOCK_GUEST_STOPPED) != 0)) {
-		src->flags &= ~PVCLOCK_GUEST_STOPPED;
+		flags &= ~PVCLOCK_GUEST_STOPPED;
+		WRITE_ONCE(src->flags, flags);
 		pvclock_touch_watchdogs();
 	}
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1ba3b7d3cae9..54458acce4f5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1832,7 +1832,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	if (use_master_clock)
 		pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
 
-	vcpu->hv_clock.flags = pvclock_flags;
+	WRITE_ONCE(vcpu->hv_clock.flags, pvclock_flags);
 
 	trace_kvm_pvclock_update(v->vcpu_id, &vcpu->hv_clock);