Message-ID: <48BAD89E.20206@colorfullife.com>
Date: Sun, 31 Aug 2008 19:45:02 +0200
From: Manfred Spraul <manfred@colorfullife.com>
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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,
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       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
References: <20080821234318.GA1754@linux.vnet.ibm.com> <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>
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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!)
> It is entirely possible that rcu_try_flip_waitack() and
> rcu_try_flip_waitmb() need to check the AND of rcu_cpu_online_map and
> cpu_online_map.  If this really is a problem (and it might well be),
> then the easiest fix is to check for cpu_is_offline(cpu) in both
> rcu_try_flip_waitmb_needed() and rcu_try_flip_waitack_needed(), and
> that in both versions of both functions.  Thoughts?
>   
I made a mistake, get_online_cpus() stores current, not a cpu number. 
Thus the described race it not possible. Perhaps there are other users 
that could deadlock.
I don't know enough about the preempt algorithm, thus I can't confirm if 
your proposal would work or not.

--
    Manfred
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