Date: Sun, 31 Aug 2008 12:23:27 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, linux-kernel@vger.kernel.org,
       cl@linux-foundation.org, mingo@elte.hu, akpm@linux-foundation.org,
       dipankar@in.ibm.com, josht@linux.vnet.ibm.com, schamp@sgi.com,
       niv@us.ibm.com, dvhltc@us.ibm.com, ego@in.ibm.com, rostedt@goodmis.org,
       peterz@infradead.org, benh@kernel.crashing.org, davem@davemloft.net,
       tony.luck@intel.com
Subject: Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU
	implementation
Message-ID: <20080831192327.GI7015@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20080825000738.GA24339@linux.vnet.ibm.com> <20080830004935.GA28548@linux.vnet.ibm.com> <48B919C2.1040809@cn.fujitsu.com> <48B94BF4.9090103@colorfullife.com> <20080830143438.GF7107@linux.vnet.ibm.com> <48BA7944.8070402@colorfullife.com> <20080831172001.GA7015@linux.vnet.ibm.com> <48BAD89E.20206@colorfullife.com> <20080831175514.GE7015@linux.vnet.ibm.com> <48BAE082.3000300@colorfullife.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <48BAE082.3000300@colorfullife.com>
User-Agent: Mutt/1.5.15+20070412 (2007-04-11)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1798
Lines: 43

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/