linux-kernel - Re: CONFIG_PREEMPT

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Date:	Sun, 9 Aug 2009 14:06:05 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CONFIG_PREEMPT_RCU in next/mmotm

On Sun, 9 Aug 2009, Hugh Dickins wrote:
> 
> filp 13343 objects:
> 
> CPU last cur F M
>   0    1   0 0 0
>   1    0   0 0 0
>   2   -1   0 0 0
>   3    0   0 0 0
> ggp = 35124, state = waitzero

Interesting that rcu_try_flip_waitzero() doesn't see 1 + 0 + -1 + 0 == 0.

That's because rcu_cpu_online_map is 0x1 instead of the 0xf it should be.

Which is because I don't have CONFIG_HOTPLUG_CPU=y on that PPC machine
(unlike the x86s), and I think you've made some recent mods which
accidentally made the rcu cpu initialization dependent on hotplug
cpu notifiers?  CONFIG_HOTPLUG_CPU=y and it works properly again.

And TREE_RCU doesn't use an rcu_cpu_online_map (though it does expect
some per-cpu initialization, but seems to get away without it).

So I think that's the mystery solved - I'll let you decide the right fix!

Hugh
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