Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code

[Oleg Nesterov <oleg@redhat.com>]

On 06/14, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > > +		spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
> >                                          ^^^^^^^^^^^^^^^^^^^^^^^
> >
> > Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand how it
> > actually works ;) But, say, rcu_check_callbacks() can be called from irq and
> > since spin_lock() doesn't increment current->rcu_read_lock_nesting this can lead
> > to rcu_preempt_qs()?
>
> No, RCU grace periods are still defined by 'heavy' context boundaries such as
> context switches, entering idle or user-space mode.
>
> PREEMPT_RCU is like traditional RCU, except that blocking is allowed within the
> RCU read critical section - that is why it uses a separate nesting counter
> (current->rcu_read_lock_nesting), not the preempt count.

Yes.

> But if a piece of kernel code is non-preemptible, such as a spinlocked region or
> an irqs-off region, then those are still natural RCU read lock regions, regardless
> of the RCU model, and need no additional RCU locking.

I do not think so. Yes I understand that rcu_preempt_qs() itself doesn't
finish the gp, but if there are no other rcu-read-lock holders then it
seems synchronize_rcu() on another CPU can return _before_ spin_unlock(),
this CPU no longer needs rcu_preempt_note_context_switch().

OK, I can be easily wrong, I do not really understand the implementation
of PREEMPT_RCU. Perhaps preempt_disable() can actually act as rcu_read_lock()
with the _current_ implementation. Still this doesn't look right even if
happens to work, and Documentation/RCU/checklist.txt says:

11.	Note that synchronize_rcu() -only- guarantees to wait until
	all currently executing rcu_read_lock()-protected RCU read-side
	critical sections complete.  It does -not- necessarily guarantee
	that all currently running interrupts, NMIs, preempt_disable()
	code, or idle loops will complete.  Therefore, if your
	read-side critical sections are protected by something other
	than rcu_read_lock(), do -not- use synchronize_rcu().

Oleg.

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