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

[Lan Tianyu]

On 2014å09æ17æ 21:10, Paul E. McKenney wrote:
> On Wed, Sep 17, 2014 at 03:11:42PM +0800, Lan Tianyu wrote:
>> On 2014å08æ29æ 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@xxxxxxxxxxxxxxxxxx>
>>> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
>>> Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
>>> Cc: Lan Tianyu <tianyu.lan@xxxxxxxxx>
>>
>> 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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