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Message-ID: <48BAE082.3000300@colorfullife.com>
Date: Sun, 31 Aug 2008 20:18:42 +0200
From: Manfred Spraul <manfred@...orfullife.com>
To: paulmck@...ux.vnet.ibm.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
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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