linux-kernel - [PATCH] x86: perf: fix incorrect use of do

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Message-Id: <20130708214404.B0B6EA66@viggo.jf.intel.com>
Date:	Mon, 08 Jul 2013 14:44:04 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org
Cc:	x86@...nel.org, Dave Hansen <dave@...1.net>
Subject: [PATCH] x86: perf: fix incorrect use of do_div() in nmi warning


From: Dave Hansen <dave.hansen@...ux.intel.com>

This fixes a bug present in 3.10 and introduced here:

commit 2ab00456ea8a0d79acb1390659b98416111880b2
Author: Dave Hansen <dave.hansen@...ux.intel.com>
Date:   Fri Jun 21 08:51:35 2013 -0700

    x86: Warn when NMI handlers take large amounts of time

I completely botched understanding the calling conventions of
do_div().  I assumed that do_div() returned the result instead
of realizing that it modifies its argument and returns a
remainder.  The side-effect from this would be bogus numbers
for the "msecs" value in the warning messages:

	INFO: NMI handler (perf_event_nmi_handler) took too long to run: 0.114 msecs

Note, there was a second fix posted by Stephane Eranian for
a separate patch which I also botched:

	http://lkml.kernel.org/r/20130704223010.GA30625@quad

Both of these fixes need to get pulled in to Linus's tree and
the 3.10 stable tree.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: stable@...r.kernel.org
Cc: x86@...nel.org
---

 linux.git-davehans/arch/x86/kernel/nmi.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff -puN arch/x86/kernel/nmi.c~dave-is-stupid-do_div arch/x86/kernel/nmi.c
--- linux.git/arch/x86/kernel/nmi.c~dave-is-stupid-do_div	2013-07-08 13:10:55.498656504 -0700
+++ linux.git-davehans/arch/x86/kernel/nmi.c	2013-07-08 13:10:55.501656637 -0700
@@ -111,7 +111,7 @@ static int __kprobes nmi_handle(unsigned
 	 */
 	list_for_each_entry_rcu(a, &desc->head, list) {
 		u64 before, delta, whole_msecs;
-		int decimal_msecs, thishandled;
+		int remainder_ns, decimal_msecs, thishandled;
 
 		before = local_clock();
 		thishandled = a->handler(type, regs);
@@ -123,8 +123,9 @@ static int __kprobes nmi_handle(unsigned
 			continue;
 
 		nmi_longest_ns = delta;
-		whole_msecs = do_div(delta, (1000 * 1000));
-		decimal_msecs = do_div(delta, 1000) % 1000;
+		whole_msecs = delta;
+		remainder_ns = do_div(whole_msecs, (1000 * 1000));
+		decimal_msecs = remainder_ns / 1000;
 		printk_ratelimited(KERN_INFO
 			"INFO: NMI handler (%ps) took too long to run: "
 			"%lld.%03d msecs\n", a->handler, whole_msecs,
_
--
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