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Subject: Re: [patch 3/9] cpu: deliver CPU_UP_CANCELED only to NOTIFY_OKed
	callbacks with CPU_UP_PREPARE
From: Rusty Russell <rusty@rustcorp.com.au>
To: Akinobu Mita <akinobu.mita@gmail.com>
Cc: linux-kernel@vger.kernel.org, Greg Kroah-Hartman <gregkh@suse.de>,
       Dmitriy Zavin <dmitriyz@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
       Andi Kleen <andi@firstfloor.org>, Ashok Raj <ashok.raj@intel.com>
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	 <20070722153349.369335694@gmail.com>
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Date: Mon, 23 Jul 2007 09:48:10 +1000
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On Mon, 2007-07-23 at 00:33 +0900, Akinobu Mita wrote:
> plain text document attachment (cpuhotplug-nr-calls.patch)
> From: Akinobu Mita <akinobu.mita@gmail.com>
> 
> The functions in a CPU notifier chain is called with CPU_UP_PREPARE event
> before making the CPU online. If one of the callback returns NOTIFY_BAD,
> it stops to deliver CPU_UP_PREPARE event, and CPU online operation is canceled.
> Then CPU_UP_CANCELED event is delivered to the functions in a CPU notifier
> chain again.
> 
> This CPU_UP_CANCELED event is delivered to the functions which have been
> called with CPU_UP_PREPARE, not delivered to the functions which haven't
> been called with CPU_UP_PREPARE.
> 
> The problem that makes existing cpu hotplug error handlings complex is
> that the CPU_UP_CANCELED event is delivered to the function that has
> returned NOTIFY_BAD, too.
> 
> Usually we don't expect to call destructor function against the
> object that has failed to initialize. It is like:
> 
> 	err = register_something();
> 	if (err) {
> 		unregister_something();
> 		return err;
> 	}
> 
> So it is natural to deliver CPU_UP_CANCELED event only to the functions
> that have returned NOTIFY_OK with CPU_UP_PREPARE event and not to call
> the function that have returned NOTIFY_BAD. This is what this patch is doing.
> 
> Otherwise, every cpu hotplug notifiler has to track whether
> notifiler event is failed or not for each cpu.
> (drivers/base/topology.c is doing this with topology_dev_map)
> 
> Similary this patch makes same thing with CPU_DOWN_PREPARE and
> CPU_DOWN_FAILED evnets.
> 
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>

This makes a great deal of sense; I consider it a bugfix.

Thanks!
Rusty.