Re: [patch V2 00/24] cpu/hotplug: Convert get_online_cpus() to a percpu_rwsem

[Mark Rutland]

On Thu, Apr 27, 2017 at 12:01:57PM +0200, Thomas Gleixner wrote:
> On Thu, 27 Apr 2017, Mark Rutland wrote:
> 
> > On Thu, Apr 27, 2017 at 10:27:20AM +0200, Sebastian Siewior wrote:
> > > On 2017-04-26 11:32:36 [+0100], Mark Rutland wrote:
> > > > > So we could end up calling static_branch_enable_cpuslocked()
> > > > > without actually holding the lock. Should we do a cpu_hotplug_begin/done in
> > > > > setup_cpu_feature_capabilities ? I agree it doesn't look that nice. Thoughts ?
> > > > 
> > > > I agree that's hideous, but it looks like the only choice given the
> > > > hotplug rwsem cahnges. :/
> > > 
> > > would work for you to provide a locked and unlocked version?
> > 
> > Maybe. Today we have:
> > 
> > // rwsem unlocked
> > start_kernel()
> > ->smp_prepare_boot_cpu()
> > -->update_cpu_errata_workarounds()
> > --->update_cpu_capabilities()
> > 
> > // rwsem locked (by other CPU)
> > secondary_start_kernel()
> > ->check_local_cpu_capabilities()
> > -->update_cpu_errata_workarounds()
> > --->update_cpu_capabilities() 
> > 
> > With the common chain:
> > 
> > update_cpu_capabilities()
> > ->cpus_set_cap()
> > -->static_branch_enable()
> > 
> > ... so we could add a update_cpu_capabilities{,_cpuslocked}(), and say
> > that cpus_set_cap() expects the hotplug rswem to be locked, as per the
> > below diff.
> 
> You just can take the rwsen in smp_prepare_boot_cpu(), so you don't need
> that conditional thingy at all. Hmm?

True.

Given it's a bit further up the callchain, it's probably worth a
comment, but it will work.

I'll spin a v3 to that effect shortly.

Thanks,
Mark.