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

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Date:   Fri, 5 Oct 2018 21:40:57 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, 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:
> 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];

Could you annotate this pointer (sbi->s_qf_names) with __rcu so it can be
checked by sparse for proper usage? Its also point #16 in the checklist.txt
RCU document. I enclosed a diff to do this below.

I also saw a bunch of places in super.c where the pointer isn't accessed from
an rcu read section or rcu_dereference, but it was a quick look so sorry if I
missed something. If its true, then are you planning to convert these to use
rcu_dereference and wrapped by an rcu_read_lock/unlock as well?

> +		to_free[i] = rcu_dereference_protected(sbi->s_qf_names[i],
> +						       &sb->s_umount);

Also should this be the following?
		to_free[i] = rcu_dereference_protected(sbi->s_qf_names[i],
					       lockdep_is_held(&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();

I had same concern as Paul here about synchronize_rcu done before the kfree.

thanks,

 - Joel

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5863fd22e90b..eec1b3090d04 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5083,7 +5083,7 @@ struct ext4_mount_options {
 	u32 s_min_batch_time, s_max_batch_time;
 #ifdef CONFIG_QUOTA
 	int s_jquota_fmt;
-	char *s_qf_names[EXT4_MAXQUOTAS];
+	char __rcu *s_qf_names[EXT4_MAXQUOTAS];
 #endif
 };