Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

[Manfred Spraul]

Paul E. McKenney wrote:
> On Sun, Aug 31, 2008 at 07:45:02PM +0200, Manfred Spraul wrote:
>   
> > Paul E. McKenney wrote:
> >     
> > > Assuming that the ordering of processing pending irqs and marking the
> > > CPU offline in cpu_online_mask can be resolved as noted above, it should
> > > work fine -- if a CPU's bit is clear, we can safely ignore it.  The race
> > > can be resolved by checking the CPU's bit in force_quiescent_state().
> > >
> > > Or am I missing something?
> > >   
> > >       
> > Yes, that would work:
> > Rule 1: after CPU_DEAD, a cpu is gone. The cpu is quiet, rcu callbacks must 
> > be moved to other cpus, ...
> > Rule 2: if a cpu is not listed in cpu_online_mask, then it can be 
> > considered as outside a read-side critical section.
> >
> > The problem with rule 2 is that it means someone [force_quiescent_state()] 
> > must poll the cpu_online_mask and look for changes.
> > I'd really prefer a notifier. CPU_DYING is nearly the correct thing, it 
> > only has to be moved down 3 lines ;-)
> > (I want to kill the bitmaps, not add a hierarchical bitmap polling system!)
> >     
>
> But some later CPU_DYING notifier might decide that the CPU cannot be
> removed after all, which would mean bringing the CPU back.  And then
> whatever the CPU was needed for might have actually happened in the
> meantime, which does not sound good to me...
>   
CPU_DYING must not fail, the current code doesn't support that.

--
   Manfred
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