From 2b4c03cbc84090174a3cf5d27887bac8fb052b57 Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Mon, 7 Sep 2015 14:37:40 +0200
Subject: kern: fix loop reading the time value

Previously, recent versions of gcc would hoist the loads out of the
loop reading the time value.

* kern/macros.h (access_once): New macro.
* kern/mach_clock.c (host_get_time): Use the new macro to prevent the
loads from being hoisted out of the loop.
---
 kern/mach_clock.c | 6 +++---
 kern/macros.h     | 2 ++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kern/mach_clock.c b/kern/mach_clock.c
index 655adf40..2bcd2325 100644
--- a/kern/mach_clock.c
+++ b/kern/mach_clock.c
@@ -426,9 +426,9 @@ host_get_time(host, current_time)
 		return(KERN_INVALID_HOST);
 
 	do {
-		current_time->seconds = mtime->seconds;
-		current_time->microseconds = mtime->microseconds;
-	} while (current_time->seconds != mtime->check_seconds);
+		current_time->seconds = access_once(mtime->seconds);
+		current_time->microseconds = access_once(mtime->microseconds);
+	} while (current_time->seconds != access_once(mtime->check_seconds));
 
 	return (KERN_SUCCESS);
 }
diff --git a/kern/macros.h b/kern/macros.h
index 7cc579d1..c2e8545a 100644
--- a/kern/macros.h
+++ b/kern/macros.h
@@ -52,6 +52,8 @@
 #define structof(ptr, type, member) \
     ((type *)((char *)(ptr) - offsetof(type, member)))
 
+#define access_once(x)      (*(volatile typeof(x) *)&(x))
+
 #define alignof(x)          __alignof__(x)
 
 #ifndef likely
-- 
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