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

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

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