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

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Message-ID: <20151216175000.GM60826@redhat.com>
Date:	Wed, 16 Dec 2015 12:50:00 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Jeff Merkey <linux.mdb@...il.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	uobergfe@...hat.com, atomlin@...hat.com, cmetcalf@...hip.com,
	fweisbec <fweisbec@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	peterz@...radead.org, luto@...nel.org
Subject: Re: [PATCH] Fix spurious hard lockup events while in debugger

On Wed, Dec 16, 2015 at 10:22:18AM -0700, Jeff Merkey wrote:
> On 12/14/15, Jeff Merkey <linux.mdb@...il.com> wrote:
> > 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
> >
> >
> 
> I got to the bottom of it.  It's related to the hardware I am using.
> One of the processors is faulting and hanging due to an existing bug
> in the hw_breakpoint handler not setting the resume flag (I have
> previously reported it and submitted a patch).  This breaks your code,
> but there's nothing you can do about it.
> 
> There is a severe bug in hw_breakpoint.c that causes int1 recursion
> and this whole "lazy debug register switching" nonsense does not work
> properly.  I am probably the first person to actually test this code
> path robustly.  I applied the patch that fixes this bug in
> hw_breakpoint.c and the problem with your code firing off and ignoring
> the touch flag
> went away.

Ah, good to know.  Thanks!  I'll drop this patch then.

Cheers,
Don
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