linux-kernel - Re: [rcu] [ INFO: suspicious RCU usage. ]

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Date:	Wed, 04 Feb 2015 17:43:44 +0100
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
	linux-kernel@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	linux-arm-kernel@...ts.infradead.org,
	Arnd Bergmann <arnd@...db.de>,
	MarkRutland <mark.rutland@....com>
Subject: Re: [rcu] [ INFO: suspicious RCU usage. ]

On śro, 2015-02-04 at 08:28 -0800, Paul E. McKenney wrote:
> On Wed, Feb 04, 2015 at 05:10:56PM +0100, Krzysztof Kozlowski wrote:
> > On śro, 2015-02-04 at 07:56 -0800, Paul E. McKenney wrote:
> > > On Wed, Feb 04, 2015 at 04:22:28PM +0100, Krzysztof Kozlowski wrote:
> > > > 
> > > > Actually the timeout versions but I think that doesn't matter.
> > > > The wait_on_bit will busy-loop with testing for the bit. Inside the loop
> > > > it calls the 'action' which in my case will be bit_wait_io_timeout().
> > > > This calls schedule_timeout().
> > > 
> > > Ah, good point.
> > > 
> > > > See proof of concept in attachment. One observed issue: hot unplug from
> > > > commandline takes a lot more time. About 7 seconds instead of ~0.5.
> > > > Probably I did something wrong.
> > > 
> > > Well, you do set the timeout to five seconds, and so if the condition
> > > does not get set before the surviving CPU finds its way to the
> > > out_of_line_wait_on_bit_timeout(), you are guaranteed to wait for at
> > > least five seconds.
> > >
> > > One alternative approach would be to have a loop around a series of
> > > shorter waits.  Other thoughts?
> > 
> > Right! That was the issue. It seems it works. I'll think also on
> > self-adapting interval as you said below. I'll test it more and send a
> > patch.
> 
> Sounds good!
> 
> Are you doing ARM, ARM64, or both?  I of course vote for both.  ;-)

I'll do both but first I need to find who has ARM64 board in my team.

Best regards,
Krzysztof



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