linux-kernel - [PATCH 2/5] x86/pvclock: no need to use strong read barriers in pvclock_get_time

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Date:	Mon,  5 Oct 2009 17:50:08 -0700
From:	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
To:	Xen-devel <xen-devel@...ts.xensource.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kurt.hackel@...cle.com,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: [PATCH 2/5] x86/pvclock: no need to use strong read barriers in pvclock_get_time_values

The time values are guaranteed to be only updated by the pcpu that the
vcpu is currently running on (preempting the guest), so there's never
any risk of cross-pcpu races.  Therefore, we only need a barrier to
prevent compiler reordering.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
---
 arch/x86/kernel/pvclock.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 963f349..5ecce7f 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -86,12 +86,12 @@ static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
 {
 	do {
 		dst->version = src->version;
-		rmb();		/* fetch version before data */
+		barrier();		/* fetch version before data */
 		dst->tsc_timestamp     = src->tsc_timestamp;
 		dst->system_timestamp  = src->system_time;
 		dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
 		dst->tsc_shift         = src->tsc_shift;
-		rmb();		/* test version after fetching data */
+		barrier();		/* test version after fetching data */
 	} while ((src->version & 1) || (dst->version != src->version));
 
 	return dst->version;
-- 
1.6.2.5

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