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

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Date:	Mon, 1 Sep 2014 13:20:59 +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 Thu, Aug 28, 2014 at 12:47:45PM -0700, 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

Please recap the actual problem; the link might die and the actual mail
linked to isn't very useful in any case.

> 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.

URGH.. I really hate that. The hotplug interface is already too
horrible, we should not add such hacks to it.

How about ripping that rcu_expedited stuff out instead? That's all
conditional anyhow, so might as well not do it.

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