Re: [PATCH 7/7] x86/mm: Switch to generic get_user_page_fast() implementation

[Paul E. McKenney]

On Thu, Mar 16, 2017 at 06:20:46PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 16, 2017 at 06:26:55PM +0300, Kirill A. Shutemov wrote:
> > +config HAVE_GENERIC_RCU_GUP
> > +	def_bool y
> > +
> 
> Nothing immediately jumped out to me; except that this option might be
> misnamed.
> 
> AFAICT that code does not in fact rely on HAVE_RCU_TABLE_FREE; it will
> happily work with the (x86) broadcast IPI invalidate model, as you show
> here.
> 
> Architectures that do not do that obviously need HAVE_RCU_TABLE_FREE,
> but that is not the point I feel.
> 
> Also, this code hard relies on IRQ-disable delaying grace periods, which
> is mostly true I think, but has always been something Paul didn't really
> want to commit too firmly to.

That is quite true!

The only case where IRQ-disable is guaranteed to delay grace periods is
when you are using RCU-sched, in other words synchronize_sched() and
call_rcu_sched().  And even then, the CPU cannot be in the idle loop,
cannot be offline, and cannot be a nohz_full CPU on its way to/from
userspace execution.

							Thanx, Paul