linux-kernel - Re: [PATCH RFC 0/5] rcu doc updates for whatisRCU and checklist

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Message-ID: <20181006053446.GA2529@thunk.org>
Date:   Sat, 6 Oct 2018 01:34:46 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>, pantin@...gle.com
Subject: Re: [PATCH RFC 0/5] rcu doc updates for whatisRCU and checklist

On Fri, Oct 05, 2018 at 08:45:40PM -0700, Paul E. McKenney wrote:
> 
> Shouldn't the synchronize_rcu() precede the loop doing the kfree()
> calls?  Or am I missing something subtle?

No, that was a cut and paste error on my part.  I was removing the
rcu_read_unlock() before the kfree loop, and accidentally removed the
synchronize_rcu().  Then when I put it back, I put it back in the
right place.

The longer version:

I originally used rcu_read_lock() and rcu_read_unlock() around setting
up to_free[] --- since whatisRCU.txt didn't talk about
rcu_derefence_proctected(), just rcu_dereference() in Section 2: "What
is RCU's Core API?"   

Then when I looked at the example in Section 3, I was surprised when I
didn't see the rcu_read_[un]lock() on the updater side, and spent some
time trying to figure out how to use rcu_dereference_protected().

Then when I did the transumation from
rcu_read_lock/rcu_dereference_protected/rcu_read_unlock to
rcu_dereference_protected, I bobbled the location of
synchronize_rcu().

						- Ted

P.S.  Pedagogically, it might make sense to show an example that only
uses the RCU core API --- I assume using rcu_read_[un]lock() and
rcu_dereference() does work; it's just non-optimal, right?  --- and
then introduce the use of rcu_dereference_protected() afterwards.