Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

[Byungchul Park]

On 3/9/2018 5:41 PM, Byungchul Park wrote:
> On Thu, Mar 08, 2018 at 10:01:56AM -0800, Paul E. McKenney wrote:
> > On Thu, Mar 08, 2018 at 07:08:25PM +0900, Byungchul Park wrote:
>
> [...]
>
> > >     2. Clear its bit of ->expmask *only* when it's out of RCU read
> > >        sections and keep others unchanged. So it will be cleared at the
> > >        end of the RCU read section in that case.
> > >
> > >        This option would also work because we anyway check both
> > >        ->exp_tasks and ->expmask to finish the expedite-gp.
> >
> > This could be made to work, but one shortcoming is that the grace
> > period would end up waiting on later read-side critical sections
> > that it does not really need to wait on.  Also, eventually all the
>
> I don't think it waits on any later ones since ->expmask would be
> cleared at the end of the previous RCU read section.

Now having simplified my patch, with the simplified version, I can
see what you were saying. Right, the expedite-gp might be extended
as you said. That optimization cannot be achieved this way without
reggression.

You're awesome. Thank you for explaning the reason why not.

--
Thanks,
Byungchul