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Date:   Tue, 6 Feb 2018 19:16:29 -0800
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     "Serge E. Hallyn" <serge@hallyn.com>
Cc:     Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: Can RCU stall lead to hard lockups?
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On Tue, Feb 06, 2018 at 08:55:04PM -0600, Serge E. Hallyn wrote:
> On Tue, Feb 06, 2018 at 06:53:37PM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 06, 2018 at 08:33:03PM -0600, Serge E. Hallyn wrote:
> > > On Sat, Feb 03, 2018 at 12:50:32PM -0800, Paul E. McKenney wrote:
> > > > On Fri, Feb 02, 2018 at 05:44:30PM -0600, Serge E. Hallyn wrote:
> > > > > Quoting Paul E. McKenney (paulmck@linux.vnet.ibm.com):
> > > > > > On Tue, Jan 09, 2018 at 06:11:14AM -0800, Tejun Heo wrote:
> > > > > > > Hello, Paul.
> > > > > > > 
> > > > > > > On Mon, Jan 08, 2018 at 08:24:25PM -0800, Paul E. McKenney wrote:
> > > > > > > > > I don't know the RCU code at all but it *looks* like the first CPU is
> > > > > > > > > taking a sweet while flushing printk buffer while holding a lock (the
> > > > > > > > > console is IPMI serial console, which faithfully emulates 115200 baud
> > > > > > > > > rate), and everyone else seems stuck waiting for that spinlock in
> > > > > > > > > rcu_check_callbacks().
> > > > > > > > > 
> > > > > > > > > Does this sound possible?
> > > > > > > > 
> > > > > > > > 115200 baud?  Ouch!!!  That -will- result in trouble from console
> > > > > > > > printing, and often also in RCU CPU stall warnings.
> > > > > > > 
> > > > > > > It could even be slower than 115200, and we occassionally see RCU
> > > > > > > stall warnings caused by printk storms, for example, while the kernel
> > > > > > > is trying to dump a lot of info after an OOM.  That's an issue we
> > > > > > > probably want to improve from printk side; however, they don't usually
> > > > > > > lead to NMI hard lockup detector kicking in and crashing the machine,
> > > > > > > which is the peculiarity here.
> > > > > > > 
> > > > > > > Hmmm... show_state_filter(), the function which dumps all task
> > > > > > > backtraces, share a similar problem and it avoids it by explicitly
> > > > > > > calling touch_nmi_watchdog().  Maybe we can do something like the
> > > > > > > following from RCU too?
> > > > > > 
> > > > > > If this fixes things for you, I would welcome such a patch.
> > > > > 
> > > > > Hi - would this also be relevant to 4.9-stable and 4.4-stable, or
> > > > > has something elsewhere changed after 4.9 that actually triggers this?
> > > > 
> > > > As far as I can tell, slow console lines have been prone to RCU CPU stall
> > > > warnings for a very long time.
> > > 
> > > Ok, thanks Paul.
> > > 
> > > Tejun were you going to push this?
> > 
> > I have it queued for the next merge window.  3eea9623926f ("rcu: Call
> > touch_nmi_watchdog() while printing stall warnings") in -rcu.
> 
> D'oh - thanks!

Not a problem at all!  Had I lost this commit, it would not have been the
first time.  ;-)

							Thanx, Paul