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

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Message-ID: <541A8698.9040905@intel.com>
Date:	Thu, 18 Sep 2014 15:15:36 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	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, 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 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?

> 
> 							Thanx, Paul
> 


-- 
Best regards
Tianyu Lan
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