Date: Fri, 17 Mar 2006 01:04:12 -0800
From: Andrew Morton <akpm@osdl.org>
To: vatsa@in.ibm.com
Cc: torvalds@osdl.org, linux-kernel@vger.kernel.org, shaohua.li@intel.com,
       bryce@osdl.org
Subject: Re: [PATCH] Check for online cpus before bringing them up
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Srivatsa Vaddagiri <vatsa@in.ibm.com> wrote:
>
> Well ..other arch-es need to have a similar check if they get around to
> implement physical hot-add (even if they allow offlining of all CPUs). This is 
> required since a user can (by mistake maybe) try to bring up an already online 
> CPU by writing a '1' to it's sysfs 'online' file. 'store_online' 
> (drivers/base/cpu.c) unconditionally calls 'smp_prepare_cpu' w/o checking for 
> this error condition. The check added in the patch catches such error 
> conditions as well.

OK..  I guess we should fix those architectures while we're thinking about it.

> --- linux-2.6.16-rc6/arch/i386/kernel/smpboot.c~cpu_hp	2006-03-17 14:27:15.000000000 +0530
> +++ linux-2.6.16-rc6-root/arch/i386/kernel/smpboot.c	2006-03-17 14:38:50.000000000 +0530
> @@ -1029,6 +1029,16 @@ int __devinit smp_prepare_cpu(int cpu)
>  	int	apicid, ret;
>  
>  	lock_cpu_hotplug();
> +
> +	/* Check if CPU is already online. This can happen if user tries to 
> +	 * bringup an already online CPU or a previous offline attempt
> +	 * on this CPU has failed.
> +	 */
> +	if (cpu_online(cpu)) {
> +		ret = -EINVAL;
> +		goto exit;
> +	}
> +

How well tested is this?  From my reading, this will cause
enable_nonboot_cpus() to panic.  Is that intended?

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