linux-kernel - [PATCH] x86: fix warning for unsynchronized TSCs (was: avoid modifying global flag tsc

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Message-ID: <4B07FDBF.3070509@hillier.de>
Date:	Sat, 21 Nov 2009 15:48:31 +0100
From:	Gernot Hillier <gernot@...lier.de>
To:	x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
CC:	linux-kernel@...r.kernel.org
Subject: [PATCH] x86: fix warning for unsynchronized TSCs (was: avoid modifying
 global flag tsc_unstable in unsychronized)

From: Gernot Hillier <gernot.hillier@...mens.com>

This is a small reminder for a patch I sent some weeks ago. 
It was already confirmed by Alok (see 
http://article.gmane.org/gmane.linux.kernel/901641). I also 
decided to give it a better summary. Here's the original 
description from last mail:

The current behavior of unsynchronized_tsc() prevents the warning for
unsychronized TSCs to be printed: tsc_init() calls unsynchronized_tsc()
first to check for such problems; if found, mark_tsc_unstable() is
called. However, the latter bails out immediately as unsynchronized_
tsc() did already set the global flag tsc_unstable. Thus, the intended
warning message "Marking TSC unstable due to TSCs unsynchronized" won't
show up.

This patch changes the behavior for unsynchronized_tsc() to only *check*
for the quirk condition, but not to modify any global flags. If I'm not
mistaken, the only place to *set* the flag shall be mark_tsc_unstable().

Signed-off-by: Gernot Hillier <gernot.hillier@...mens.com>
Reviewed-by: Jan Kiszka <jan.kiszka@...mens.com>
Reviewed-by: Wolfgang Mauerer <wolfgang.mauerer@...mens.com>
Reviewed-by: Alok Kataria <akataria@...are.com>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index cd982f4..ffbb786 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -836,10 +836,10 @@ __cpuinit int unsynchronized_tsc(void)
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
 		/* assume multi socket systems are not synchronized: */
 		if (num_possible_cpus() > 1)
-			tsc_unstable = 1;
+			return 1;
 	}

-	return tsc_unstable;
+	return 0;
 }

 static void __init init_tsc_clocksource(void)
--
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