linux-kernel - Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

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Message-ID: <20160131002825.GJ6719@linux.vnet.ibm.com>
Date:	Sat, 30 Jan 2016 16:28:25 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

On Fri, Jan 29, 2016 at 04:27:35PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 15, 2016 at 03:14:10PM -0800, Paul E. McKenney wrote:
> > And if I make the scheduling-clock interrupt send extra wakeups to the RCU
> > grace-period kthread when needed, things work even with CPU hotplug going.
> > 
> > The "when needed" means any time that the RCU grace-period kthread has
> > been sleeping three times as long as the timeout interval.  If the first
> > wakeup does nothing, it does another wakeup once per second.
> > 
> > So it looks like this change makes an existing problem much worse, as
> > opposed to introducing a new problem.
> 
> I have a vague idea about a possible race window. Have you been
> observing this on PPC or x86?
> 
> The reason I'm asking is that PPC (obviously) allows for more races :-)

;-)

I have been seeing this on x86.

							Thanx, Paul