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Date:	Fri, 19 Sep 2014 00:55:11 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Lan Tianyu <tianyu.lan@...el.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods

On Thursday, September 18, 2014 05:38:45 AM Paul E. McKenney wrote:
> On Thu, Sep 18, 2014 at 03:15:36PM +0800, Lan Tianyu wrote:
> > On 2014年09月17日 21:10, Paul E. McKenney wrote:
> > > On Wed, Sep 17, 2014 at 03:11:42PM +0800, Lan Tianyu wrote:
> > >> On 2014年08月29日 03:47, Paul E. McKenney wrote:
> > >>> Currently, the expedited grace-period primitives do get_online_cpus().
> > >>> This greatly simplifies their implementation, but means that calls to
> > >>> them holding locks that are acquired by CPU-hotplug notifiers (to say
> > >>> nothing of calls to these primitives from CPU-hotplug notifiers) can
> > >>> deadlock.  But this is starting to become inconvenient:
> > >>> https://lkml.org/lkml/2014/8/5/754
> > >>>
> > >>> This commit avoids the deadlock and retains the simplicity by creating
> > >>> a try_get_online_cpus(), which returns false if the get_online_cpus()
> > >>> reference count could not immediately be incremented.  If a call to
> > >>> try_get_online_cpus() returns true, the expedited primitives operate
> > >>> as before.  If a call returns false, the expedited primitives fall back
> > >>> to normal grace-period operations.  This falling back of course results
> > >>> in increased grace-period latency, but only during times when CPU
> > >>> hotplug operations are actually in flight.  The effect should therefore
> > >>> be negligible during normal operation.
> > >>>
> > >>> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > >>> Cc: Josh Triplett <josh@...htriplett.org>
> > >>> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> > >>> Cc: Lan Tianyu <tianyu.lan@...el.com>
> > >>
> > >> Hi Paul:
> > >> 	What's the status of the patch? Will you push it? Thanks.
> > > 
> > > By default, it would go into 3.19.  Do you need it earlier?
> > 
> > IMO, this is a dead lock bug which is hard to reproduce and the patch
> > should go into v3.17 and stable tree?
> 
> The problem with pushing for v3.17 is that I would have to rebase
> that commit to the bottom of my current stack and redo all my testing.
> If there were any problems, I could not only miss v3.17, but also miss
> the v3.18 merge window.
> 
> So, given that the next merge window happens pretty soon, how about
> v3.18 and the stable tree?

That sounds good to me.

Rafael

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