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]; > + 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. Also I was wondering if the "if (sbi->s_qf_names[USRQUOTA])" in the patch should be "if (rcu_dereference(sbi->s_qf_names[USRQUOTA]))". I don't think the compiler could optimize the access in this case, bit IMO using the rcu_dereference would serve to document that its an RCU protected pointer anyway. thanks, - Joel
