linux-kernel - Re: [PATCH RT] rt: Make cpu_chill() use hrtimer instead of msleep()

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Message-ID: <52F4EBFA.3040401@linutronix.de>
Date:	Fri, 07 Feb 2014 15:21:46 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	John Kacur <jkacur@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>
Subject: Re: [PATCH RT] rt: Make cpu_chill() use hrtimer instead of msleep()

On 02/07/2014 03:13 PM, Steven Rostedt wrote:
> On Fri, 7 Feb 2014 09:08:34 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> 
>>> could you please tell me which two locks are invovled here?
> 
> I should have also stated that it was only one lock that was involved.
> But the lock owner was doing a msleep() that requires a wakeup by
> ksoftirqd to continue. If ksoftirqd happens to be blocked on a lock
> held by the msleep() caller, then you have your deadlock.

That makes sense.

> It's best not to have any softirqs going to sleep requiring another
> softirq to wake it up. Note, if we ever require a timer softirq to do a
> cpu_chill() it will most definitely hit this deadlock.

Yes. And that sleep in softirq is also not really nice but this isn't
new. Thanks for the patch & explanation.
> 
> -- Steve

Sebastian
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