Re: [QUESTION] srcu: Remove the SCAN2 state

[Byungchul Park]

On 2/22/2018 11:11 AM, Paul E. McKenney wrote:
> On Thu, Feb 22, 2018 at 08:57:27AM +0900, Byungchul Park wrote:
> > Hello,
> >
> > I'm sorry for bothering you, and I seem to be obviously missing
> > something, but I'm really wondering why we check try_check_zero()
> > again in the state, SCAN1, for the previous srcu_idx.
> >
> > I mean, since we've already checked try_check_zero() in the previous
> > grace period and gotten 'true' as a return value, all readers who see
> > the flipped idx via srcu_flip() won't update the src_{lock,unlock}_count
> > for the previous idx until it gets flipped back again.
> >
> > Is there any reasons we check try_check_zero() again in the state, SCAN1?
> > Is there any problems if the following patch's applied?
>
> Indeed there are!  Removing the second scan exposes us to a nasty race
> condition where a reader is preempted (or interrupted or whatever) just

Indeed! I missed the cases. It should be as it is.

Thanks a lot for pointing it out.

> after fetching its counter.  A detailed explanation for an essentially

--
Thanks,
Byungchul