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

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Message-ID: <20140917131013.GU4723@linux.vnet.ibm.com>
Date:	Wed, 17 Sep 2014 06:10:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lan Tianyu <tianyu.lan@...el.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, 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, "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug
 and expedited grace periods

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?

							Thanx, Paul

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