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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Cc: Lan Tianyu <tianyu.lan@intel.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