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

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Message-ID: <CAEXW_YTSW9kr3DsJm6hTQ0FfwVbVjzDa8=7H29+ysD10ZCbnHA@mail.gmail.com>
Date:   Thu, 15 Dec 2022 17:48:46 +0000
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 5:08 PM Paul E. McKenney <paulmck@...nel.org> 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