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    DateFri, 17 Mar 2006 14:16:53 +0530
    FromSrivatsa Vaddagiri <>
    SubjectRe: [PATCH] Check for online cpus before bringing them up
    On Thu, Mar 16, 2006 at 05:08:14PM -0800, Andrew Morton wrote:
    > Is x86 the only architecture which is exposed to this?
    
    Currently only x86 implements smp_prepare_cpu(). On other arch, it is a
    no-op. Hence yes, only x86 is exposed to this bug.
    
    > > 
    > >  	lock_cpu_hotplug();
    > > +
    > > +	if (cpu_online(cpu)) {
    > > +		ret = -EINVAL;
    > > +		goto exit;
    > > +	}
    > > +
    > >  	apicid = x86_cpu_to_apicid[cpu];
    > >  	if (apicid == BAD_APICID) {
    > >  		ret = -ENODEV;
    > 
    > a) It's hard for the reader to understand what that test is doing there
    > 
    > b) People copy code from x86, so other architectures which are not
    >    exposed to this problem will end up having a pointless test in there.
    
    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.
    
    > IOW: please comment your code.   I'll fix this one up.
    
    Sorry about not commenting my code earlier! How does the patch below look?
    
    
    Add check for online cpus.
    
    Signed-off-by : Srivatsa Vaddagiri <vatsa@in.ibm.com>
    
    
     arch/i386/kernel/smpboot.c |   10 ++++++++++
     1 files changed, 10 insertions(+)
    diff -puN arch/i386/kernel/smpboot.c~cpu_hp arch/i386/kernel/smpboot.c
    --- 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;
    +	}
    +
     	apicid = x86_cpu_to_apicid[cpu];
     	if (apicid == BAD_APICID) {
     		ret = -ENODEV;
    _
    
    -- 
    Regards,
    vatsa
    -
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