Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check() comments

[Joel Fernandes]

On Thu, Dec 15, 2022 at 5:48 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 15, 2022 at 5:08 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> > > Scenario for the reader to increment the old idx once:
> > >
> > > _ Assume ssp->srcu_idx is initially 0.
> > > _ The READER reads idx that is 0
> > > _ The updater runs and flips the idx that is now 1
> > > _ The reader resumes with 0 as an index but on the next srcu_read_lock()
> > >   it will see the new idx which is 1
> > >
> > > What could be the scenario for it to increment the old idx twice?
> >
> > Unless I am missing something, the reader must reference the
> > srcu_unlock_count[old_idx] and then do smp_mb() before it will be
> > absolutely guaranteed of seeing the new value of ->srcu_idx.
>
> I think both of you are right depending on how the flip raced with the
> first reader's unlock in that specific task.
>
> If the first read section's srcu_read_unlock() and its corresponding
> smp_mb()  happened before the flip, then the increment of old idx
> would happen only once. The next srcu_read_lock() will read the new
> index. If the srcu_read_unlock() and it's corresponding smp_mb()
> happened after the flip, the old_idx will be sampled again and can be
> incremented twice. So it depends on how the flip races with
> srcu_read_unlock().

I am sorry this is inverted, but my statement's gist stands I believe:

1. Flip+smp_mb() happened before unlock's smp_mb() -- reader will not
increment old_idx the second time.

2. unlock()'s smp_mb() happened before Flip+smp_mb() , now the reader
has no new smp_mb() that happens AFTER the flip happened. So it can
totally sample the old idx again -- that particular reader will
increment twice, but the next time, it will see the flipped one.

Did I get that right? Thanks.