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
};
