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

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Message-ID: <20080831192327.GI7015@linux.vnet.ibm.com>
Date:	Sun, 31 Aug 2008 12:23:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, mingo@...e.hu, akpm@...ux-foundation.org,
	dipankar@...ibm.com, josht@...ux.vnet.ibm.com, schamp@....com,
	niv@...ibm.com, dvhltc@...ibm.com, ego@...ibm.com,
	rostedt@...dmis.org, peterz@...radead.org,
	benh@...nel.crashing.org, davem@...emloft.net, tony.luck@...el.com
Subject: Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU
	implementation

On Sun, Aug 31, 2008 at 08:18:42PM +0200, Manfred Spraul wrote:
> 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.

Good point!

							Thanx, Paul
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