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

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Message-ID: <20160129152735.GB407@worktop>
Date:	Fri, 29 Jan 2016 16:27:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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 :-)