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

[Joel Fernandes]

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().

Also, since this is all hard to reason about I started making some
diagrams, LOL. For your amusement, here is why need to scan both idx
during grace period detection: https://i.imgur.com/jz4bNKd.png

thanks,

 - Joel