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 >
