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yaml --- r: 251979 b: refs/heads/master c: 057e6a8 h: refs/heads/master i: 251977: b21c068 251975: d2872de v: v3
git-mirror · May 24, 2011 · 3fb95e3 · 3fb95e3
1 parent ac03907
commit 3fb95e3
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Showing 2 changed files with 6 additions and 5 deletions.
diff --git a/[refs] b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 8c49d9a74bac5ea3f18480307057241b808fcc0c
+refs/heads/master: 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d
diff --git a/trunk/arch/x86/kernel/tsc.c b/trunk/arch/x86/kernel/tsc.c
@@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void)
 	cycle_t ret;
 
 	/*
-	 * Surround the RDTSC by barriers, to make sure it's not
-	 * speculated to outside the seqlock critical section and
-	 * does not cause time warps:
+	 * Empirically, a fence (of type that depends on the CPU)
+	 * before rdtsc is enough to ensure that rdtsc is ordered
+	 * with respect to loads.  The various CPU manuals are unclear
+	 * as to whether rdtsc can be reordered with later loads,
+	 * but no one has ever seen it happen.
 	 */
 	rdtsc_barrier();
 	ret = (cycle_t)vget_cycles();
-	rdtsc_barrier();
 
 	return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
 		ret : VVAR(vsyscall_gtod_data).clock.cycle_last;