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

[Manfred Spraul <manfred@colorfullife.com>]

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