Re: [Xen-devel] [PATCH RFC 01/49] xen/sched: call cpu_disable_scheduler() via cpu notifier

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 4/1/19 10:40 AM, Juergen Gross wrote:
> On 01/04/2019 11:21, Julien Grall wrote:
> > Hi,
> >
> > On 3/29/19 3:08 PM, Juergen Gross wrote:
> > > cpu_disable_scheduler() is being called from __cpu_disable() today.
> > > There is no need to execute it on the cpu just being disabled, so use
> > > the CPU_DEAD case of the cpu notifier chain. Moving the call out of
> > > stop_machine() context is fine, as we just need to hold the domain RCU
> > > lock and need the scheduler percpu data to be still allocated.
> > >
> > > Add another hook for CPU_DOWN_PREPARE to bail out early in case
> > > cpu_disable_scheduler() would fail. This will avoid crashes in rare
> > > cases for cpu hotplug or suspend.
> > >
> > > While at it remove a superfluous smp_mb() in the ARM __cpu_disable()
> > > incarnation.
> > This is not obvious why the smp_mb() is superfluous. Can you please
> > provide more details on why this is not necessary?
> cpumask_clear_cpu() should already have the needed semantics, no?
> It is based on clear_bit() which is defined to be atomic.
atomicity does not mean the store/load cannot be re-ordered by the CPU. 
You would need a barrier to prevent re-ordering.
cpumask_clear_cpu() and clear_bit() does not contain any barrier, so 
store/load can be re-ordered.
I see we have similar smp_mb() barrier in __cpu_die(). Sadly, there are 
no documentation in the code why the barrier is here. The logs don't 
help either.
The barrier here will ensure that the load/store related to disabling 
the CPU are seen before any load/store happening after the return. 
Although, I am not sure why this is necessary.
Stefano, Do you remember the rationale?

Cheers,

--
Julien Grall

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