linux-kernel - [081/114] time: Workaround gcc loop optimization that causes 64bit div errors

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Message-Id: <20100824224508.268146014@clark.site>
Date:	Tue, 24 Aug 2010 15:45:26 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	John Stultz <johnstul@...ibm.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Larry Finger <Larry.Finger@...inger.net>,
	Ingo Molnar <mingo@...e.hu>
Subject: [081/114] time: Workaround gcc loop optimization that causes 64bit div errors

2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------

From: John Stultz <johnstul@...ibm.com>

commit c7dcf87a6881bf796faee83003163eb3de41a309 upstream.

Early 4.3 versions of gcc apparently aggressively optimize the raw
time accumulation loop, replacing it with a divide.

On 32bit systems, this causes the following link errors:
	undefined reference to `__umoddi3'
	undefined reference to `__udivdi3'

The gcc issue has been fixed in 4.4 and greater.

This patch replaces the accumulation loop with a do_div, as suggested
by Linus.

Signed-off-by: John Stultz <johnstul@...ibm.com>
CC: Jason Wessel <jason.wessel@...driver.com>
CC: Larry Finger <Larry.Finger@...inger.net>
CC: Ingo Molnar <mingo@...e.hu>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/time/timekeeping.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -756,9 +756,10 @@ static cycle_t logarithmic_accumulation(
 	/* Accumulate raw time */
 	raw_nsecs = timekeeper.raw_interval << shift;
 	raw_nsecs += raw_time.tv_nsec;
-	while (raw_nsecs >= NSEC_PER_SEC) {
-		raw_nsecs -= NSEC_PER_SEC;
-		raw_time.tv_sec++;
+	if (raw_nsecs >= NSEC_PER_SEC) {
+		u64 raw_secs = raw_nsecs;
+		raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);
+		raw_time.tv_sec += raw_secs;
 	}
 	raw_time.tv_nsec = raw_nsecs;
 


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