linux-kernel - Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods

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Message-ID: <20140901161735.GA5806@worktop.ger.corp.intel.com>
Date:	Mon, 1 Sep 2014 18:17:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	<""@rjwysocki.net>, tianyu.lan@...el.com
Subject: Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug
 and expedited grace periods

On Mon, Sep 01, 2014 at 09:05:50AM -0700, Paul E. McKenney wrote:
> > URGH.. I really hate that. The hotplug interface is already too
> > horrible, we should not add such hacks to it.
> 
> We do have try_ interfaces to a number of other subsystems, so I don't
> believe that it qualifies as such a hack.

We do indeed, but I'm not sure about adding this to the hotplug stuff.

Also; not really understanding the problem doesn't help. 

> > How about ripping that rcu_expedited stuff out instead? That's all
> > conditional anyhow, so might as well not do it.
> 
> In what way is the expedited stuff conditional?

synchronize_sched() conditionally calls synchronize_sched_expedited()
and its condition: rcu_expedited, gets set/cleared on pm notifiers and
nr_cpu_ids.


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