在 2013-04-29一的 10:00 +0530，Srivatsa S. Bhat写道：
> On 04/29/2013 08:19 AM, liguang wrote:
> > in cpu_down(), _cpu_down() will do
> > "
> > if (num_online_cpus() == 1)
> > return -EBUSY;
> > "
> > when cpu_hotplug_disabled was set, num_online_cpus
> > will return 1 for there's only 1 boot cpu.
> > so, it's unnecessary to check cpu_hotplug_disabled
> > here.
> >
>
> The 2 checks serve very different purposes; they are not the same!
purposes are different, but I think effects are same for this case,
the statement 'if ((num_online_cpus() == 1)' seems
have same effect with cpu_hotplug_disabled here,
because when cpu_hotplug_disabled, only boot cpu is online
>
> The num_online_cpus() check is to ensure that the user doesn't do
> something insane like trying to offline the last online CPU in the
> system.
>
> Whereas, the flag 'cpu_hotplug_disabled' is used to prevent user-
> triggered CPU hotplug (such as those initiated through sysfs).
> This is useful in cases where the system itself wants to initiate CPU
> hotplug and it doesn't want annoying races with CPU hotplug going
> on in parallel due to other reasons. One such case is suspend/resume.
> That's why, if you have noticed, the suspend/resume code invokes the
> _cpu_down() version, in order to bypass the flag and get its job done.
>
> So, no, I think the check needs to stay.
>
> Regards,
> Srivatsa S. Bhat
>
> > Signed-off-by: liguang <lig.f...@cn.fujitsu.com>
> > ---
> > kernel/cpu.c | 6 ------
> > 1 files changed, 0 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index b5e4ab2..cd166d3 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -330,14 +330,8 @@ int __ref cpu_down(unsigned int cpu)
> >
> > cpu_maps_update_begin();
> >
> > - if (cpu_hotplug_disabled) {
> > - err = -EBUSY;
> > - goto out;
> > - }
> > -
> > err = _cpu_down(cpu, 0);
> >
> > -out:
> > cpu_maps_update_done();
> > return err;
> > }
> >
>
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