linux-kernel - [PATCH] Fix spurious hard lockup events while in debugger

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Date:	Mon, 14 Dec 2015 20:30:45 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	akpm@...ux-foundation.org, uobergfe@...hat.com, dzickus@...hat.com,
	atomlin@...hat.com, cmetcalf@...hip.com, fweisbec@...il.com
Subject: [PATCH] Fix spurious hard lockup events while in debugger

The current touch_nmi_watchdog() function in /kernel/watchdog.c does
not always catch all cases when a processor is spinning in the nmi
handler inside either KGDB, KDB, or MDB, in particular, the case where
a processor is being held by a debugger inside an int1 handler.

The hrtimer_interrupts_saved count can still end up matching the
hrtime value in some cases, resulting in the hard lockup detector
tagging processors inside a debugger and executing a panic.

The patch below corrects this problem.  I did not add this to
the touch_nmi_function directly becuase of possible affects on
timing issues since the function is widely used by drivers and
modules.

I have tested this patch and it fixes the problem for kernel debuggers
stopping errant hard lockup events when processors are spinning inside
the debugger.

Signed-off-by: Jeff Merkey <linux.mdb@...il.com>
---
 kernel/watchdog.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 18f34cf..b682aab 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -283,6 +283,13 @@ static bool is_hardlockup(void)
 	__this_cpu_write(hrtimer_interrupts_saved, hrint);
 	return false;
 }
+
+void touch_hardlockup_watchdog(void)
+{
+	__this_cpu_write(hrtimer_interrupts_saved, 0);
+}
+EXPORT_SYMBOL_GPL(touch_hardlockup_watchdog);
+
 #endif

 static int is_softlockup(unsigned long touch_ts)
-- 
1.8.3.1

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