Re: [PATCH] x86/hyperv: Remove unregister syscore call from hyperv cleanup

[Wei Liu <wei.liu@kernel.org>]

On Fri, Nov 25, 2022 at 09:09:52PM +0530, Gaurav Kohli wrote:
> 
> On 11/25/2022 8:58 PM, Wei Liu wrote:
> > On Wed, Nov 23, 2022 at 09:23:11PM -0800, Gaurav Kohli wrote:
> > > Hyperv cleanup codes comes under panic path where preemption and irq
> > Please use "Hyper-V" throughout.
> Thanks for the comment, sure will do on v2.
> > 
> > > is already disabled. So calling of unregister_syscore_ops which has mutex
> > > from hyperv cleanup might schedule out the thread and never comes back.
> > > 
> > While on paper this looks problematic -- have you seen this issue
> > triggered in real life?
> > 
> > This looks to be only triggered when there is another thread already
> > holding the mutex, which seems rather rare in the life cycle of the
> > machine?
> 
> 
> Earlier we also suspected the same that someone was holding the lock, but
> actually there
> 
> was no owner of lock and it got scheduled out due to might sleep code in
> mutex_lock.
> 
> Looks like where voluntary preemption config is on, there it is getting
> scheduled out in might sleep.

If there is only one CPU online and the mutex is free, then
rescheduling will not have any adverse effect, right? Does it not get
scheduled on the only available CPU and make further progress?

> 
> But there is no need of unregister_syscore_ops as this is in crash path
> only, So removing the same.

I'm not against removing it, but the reasoning left in the commit
message and comment should reflect what happens.

Thanks,
Wei.