linux-kernel - Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog and touch_softlockup

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Message-ID: <20100820025749.GB4879@redhat.com>
Date:	Thu, 19 Aug 2010 22:57:49 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Len Brown <len.brown@...el.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org, Andy Grover <andy.grover@...cle.com>
Subject: Re: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog
 and touch_softlockup_watchdog

On Wed, Aug 18, 2010 at 01:01:56PM -0700, Andrew Morton wrote:
> The surprise new requirement that touch_nmi_watchdog() be called from
> non-preemptible code does seem to make sense IMO.  It's hard to see why
> anyone would be touching the watchdog unless he's spinning in irqs-off
> code.  Except, of course, when we have a utility function which can be
> called from wither irqs-on or irqs-off: acpi_os_stall().
> 
> That being said, it's not good to introduce new API requirements by
> accident!  An audit of all callers should first be performed, at least.
> 
> 
> The surprise new requirement that touch_softlockup_watchdog() be called
> from non-preemptible code doesn't make sense IMO.  If I have a piece of
> code in the kernel which I expect to sit in TASK_UNINTERRUPTIBLE state
> for three minutes waiting for my egg to boil, I should be able to do
> that and I should be able to touch the softlockup detector without
> needing to go non-preemptible.

Ok, so here is my patch that syncs the touch_*_watchdog back in line with
the old semantics.  Hopefully this will undo any harm I caused.

------------cut -->---------------------------

>From b372e821c804982438db090db6b4a2f753c78091 Mon Sep 17 00:00:00 2001
From: Don Zickus <dzickus@...hat.com>
Date: Thu, 19 Aug 2010 22:48:26 -0400
Subject: [PATCH] [lockup detector] sync touch_*_watchdog back to old semantics

During my rewrite, the semantics of touch_nmi_watchdog and
touch_softlockup_watchdog changed enough to break some drivers
(mostly over preemptable regions).

This change brings those touch_*_watchdog functions back in line
to how they used to work.

Signed-off-by: Don Zickus <dzickus@...hat.com>
---
 kernel/watchdog.c |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 613bc1f..99e35a2 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -122,7 +122,7 @@ static void __touch_watchdog(void)
 
 void touch_softlockup_watchdog(void)
 {
-	__get_cpu_var(watchdog_touch_ts) = 0;
+	__raw_get_cpu_var(watchdog_touch_ts) = 0;
 }
 EXPORT_SYMBOL(touch_softlockup_watchdog);
 
@@ -142,7 +142,14 @@ void touch_all_softlockup_watchdogs(void)
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 void touch_nmi_watchdog(void)
 {
-	__get_cpu_var(watchdog_nmi_touch) = true;
+	if (watchdog_enabled) {
+		unsigned cpu;
+
+		for_each_present_cpu(cpu) {
+			if (per_cpu(watchdog_nmi_touch, cpu) != true)
+				per_cpu(watchdog_nmi_touch, cpu) = true;
+		}
+	}
 	touch_softlockup_watchdog();
 }
 EXPORT_SYMBOL(touch_nmi_watchdog);
@@ -430,6 +437,9 @@ static int watchdog_enable(int cpu)
 		wake_up_process(p);
 	}
 
+	/* if any cpu succeeds, watchdog is considered enabled for the system */
+	watchdog_enabled = 1;
+
 	return 0;
 }
 
@@ -452,9 +462,6 @@ static void watchdog_disable(int cpu)
 		per_cpu(softlockup_watchdog, cpu) = NULL;
 		kthread_stop(p);
 	}
-
-	/* if any cpu succeeds, watchdog is considered enabled for the system */
-	watchdog_enabled = 1;
 }
 
 static void watchdog_enable_all_cpus(void)
-- 
1.7.2.1


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