Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable

[Frederic Weisbecker <frederic@kernel.org>]

On Tue, Jul 27, 2021 at 09:33:42PM +0200, Thomas Gleixner wrote:
> On Tue, Jul 27 2021 at 10:23, Paul E. McKenney wrote:
> > On Tue, Jul 27, 2021 at 06:38:15PM +0200, Sebastian Andrzej Siewior wrote:
> >> One thing that has been overseen is that a task within a migrate-disable
> >> region (as on PREEMPT_RT with disabled BH) is fully preemptible but may
> >> not be migrated to another CPU which should be enough to guarantee that
> >> rdp remains stable.
> >> 
> >> Check also disabled migration of the task if the RCU data pointer is
> >> from current CPU. Put the whole check within an SMP ifdef block since
> >> without SMP there are not CPU migrations to worry about (also
> >> task_struct::migration_disabled is missing).
> >> 
> >> Cc: Frederic Weisbecker <frederic@kernel.org>
> >> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> >> ---
> >> I don't fully understand why the CPU-hotplug lock matters here but this
> >> is beside the point ;)
> >
> > If I remember correctly, any attempt to change the offloaded state
> > must hold off CPU-hotplug operations.  So if the current thread is
> > holding off CPU-hotplug operations, no other thread can be doing
> > an offload or de-offload operation.
> 
> It only prevents unplugging of a CPU, but not plugging a CPU.

Hmm, but both _cpu_down() and _cpu_up() do cpus_write_lock().
What did I overlook?

PS: just had a quick look and no RCU cpu up operation seem to even check
RCU nocb offload state. So we should be fine ( -ENOSPC for further
famous last words to engrave).

Thanks.