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

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Message-ID: <20140901160550.GL5001@linux.vnet.ibm.com>
Date:	Mon, 1 Sep 2014 09:05:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 01:20:59PM +0200, Peter Zijlstra wrote:
> 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.

Will do.

> > 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.

We do have try_ interfaces to a number of other subsystems, so I don't
believe that it qualifies as such a hack.

> 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?

							Thanx, Paul

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