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SubjectRe: [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]

Very cool!

One question... In the following hunk:


@@ -5353,9 +5362,13 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
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();
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

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