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

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Message-Id: <20181006034540.GM2674@linux.ibm.com>
Date:   Fri, 5 Oct 2018 20:45:40 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        "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 07:46:28PM -0400, Theodore Y. Ts'o wrote:
> On Fri, Oct 05, 2018 at 04:18:09PM -0700, Joel Fernandes (Google) wrote:
> > 
> > Here are this week's rcu doc updates based on combing through whatisRCU and
> > checklists. Hopefully you agree with them. I left several old _bh and _sched
> > API references as is, since I don't think its a good idea to remove them till
> > the APIs themselves are removed, however I did remove several of them as well
> > (like in the first patch in this series) since I feel its better to "encourage"
> > new users not to use the old API.
> 
> Hi Joel,
> 
> As it so happens, I just recently wrote my first RCU patch[1] (file
> systems, especially on-disk data structures, generally tend not to be
> good candidates for RCU semantics).
> 
> [1] http://patchwork.ozlabs.org/patch/979779/

Very cool!

One question...  In the following hunk:

------------------------------------------------------------------------

@@ -5353,9 +5362,13 @@  static int ext4_remount(struct super_block *sb, int *flags, char *data)
 #ifdef CONFIG_QUOTA
 	sbi->s_jquota_fmt = old_opts.s_jquota_fmt;
 	for (i = 0; i < EXT4_MAXQUOTAS; i++) {
-		kfree(sbi->s_qf_names[i]);
-		sbi->s_qf_names[i] = old_opts.s_qf_names[i];
+		to_free[i] = rcu_dereference_protected(sbi->s_qf_names[i],
+						       &sb->s_umount);
+		rcu_assign_pointer(sbi->s_qf_names[i], old_opts.s_qf_names[i]);
 	}
+	for (i = 0; i < EXT4_MAXQUOTAS; i++)
+		kfree(to_free[i]);
+	synchronize_rcu();
 #endif
 	kfree(orig_data);
 	return err;

------------------------------------------------------------------------

Shouldn't the synchronize_rcu() precede the loop doing the kfree()
calls?  Or am I missing something subtle?

Otherwise, looks good!  I was worried that seq_show_option() might
sleep, but it looks like it is just putting characters into an
array.  If there is lingering concern, CONFIG_PROVE_LOCKING will
usually catch that sort of thing.

							Thanx, Paul

> So if you are working on improving RCU documentation, I thought I
> would give two comments on the RCU docs from the perspective of a
> developer trying to use RCU for the first time.
> 
> * whatisRCU is great, but one the example in Section 3 uses
>   rcu_dereference_protected() without explaining it.  Given that using
>   that function seems to be considered best practice, maybe a few more
>   words there would be in order?  That function isn't mentioned in
>   rcu.txt either, BTW.
> 
> * lockdep.txt *does* explain what rcu_dereference_protected() does,
>   but it doesn't really describe lockdep_is_held().  You can mostly
>   figure it out from context, but it wasn't obvious to me what locks
>   it could be used against, and in the case of a rw_semaphore, whether
>   it applied to shared as well as exclusive locks.  That's a lockdep
>   abstraction, and not a RCU abstraction, but lockdep isn't
>   particularly well documented, so I ended up spending 20-30 minutes
>   or so looking at the lockdep implementation before I was sure it
>   actually worked the way I thought it was going to.
> 
> Anyway, I was going to put submitting a patch to improve whatisRCU on
> my (vastly over-long) TODO list, but when I saw your patch set, I
> couldn't resist trying to see if I could fob it off on you.  If you
> don't think that's fair (and it probably isn't really), just let me
> know, and I'll put it back on my todo list.  :-)
> 
> Cheers,
> 
> 					- Ted
>