linux-kernel - Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx

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Date:   Thu, 15 Dec 2022 15:33:39 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>, boqun.feng@...il.com,
        neeraj.iitr10@...il.com, urezki@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check()
 comments

On Thu, Dec 15, 2022 at 3:03 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> Hi Paul,
>
> On Thu, Dec 15, 2022 at 2:58 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> [...]
> > > 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 do understand that a number of people like reasoning about
> > memory-barrier ordering, courtesy of the sequentially consistent portions
> > of the C and C++ memory models, but thinking in terms of the accesses
> > surrounding the memory barriers has been far less error-prone.
>
> Sure, but we are already talking in terms of the access to idx right?
> That's what we're saying is visible by memory barriers and we are
> trying to reason here about the ordering (flip does the write to idx
> and followed by smp_mb(), and there is corresponding read of idx on
> the srcu_read_lock() side. So we are indeed talking in terms of
> access, but let me know if I missed something.
>
> > > 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
> >
> > Nice!
> >
> > I suggest placing a gap between GP 2 and GP 3.  That way, you can make it
> > very clear that Reader 1's critical section starts after the end of GP 2
> > (thus clearly never blocking GP 2) and before GP 3 (thus possibly having
> > a reference to some data that is going to be freed at the end of GP 3).
> >
> > I also suggest coloring Reader 1 red and Reader 2 green, given that the
> > color red generally indicates danger.
>
> Thanks for these suggestions! I will make the update. I am planning to
> make a number of diagrams for other scenarios as well, as it helps
> visualize. Google drawing is nice for these. I am happy to share these
> with you all if there is interest :).

I made these updates, please see: https://i.imgur.com/hoKLvtt.png

Feel free to use the image for any purpose and thanks ;-)

 - Joel