Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter()

[John Stultz <john.stultz@linaro.org>]

On Mon, Jul 25, 2016 at 10:01 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> Paul, sorry for delay.
>
> On 07/21, Paul E. McKenney wrote:
>>
>> On Thu, Jul 21, 2016 at 07:34:36PM +0200, Oleg Nesterov wrote:
>> > On 07/20, Paul E. McKenney wrote:
>> > >
>> > > On Wed, Jul 20, 2016 at 07:16:03PM +0200, Oleg Nesterov wrote:
>> > >
>> > > > Now, suppose we add the additional enter/exit's:
>> > > >
>> > > >         freeze_super(sb)
>> > > >         {
>> > > >                 // this doesn't block
>> > > >                 __rcu_sync_enter(SEM3);
>> > > >                 __rcu_sync_enter(SEM2);
>> > > >                 __rcu_sync_enter(SEM1);
>> > > >
>> > > >                 down_write(&sb->s_umount);
>> > > >                 if (NEED_TO_FREEZE) {
>> > > >                         percpu_down_write(SEM1);
>> > >
>> > > The above waits for the grace period initiated by __rcu_sync_enter(),
>> > > correct?  Presumably "yes", because it will invoke rcu_sync_enter(), which
>> > > will see the state as GP_ENTER, and will thus wait.
>> >
>> > But if down_write() blocks and/or NEED_TO_FREEZE takes some time it
>> > could already see the GP_PASSED state, or at least it can sleep less.
>> >
>> > > But your point is that if !NEED_TO_FREEZE, we will get here without
>> > > waiting for a grace period.
>> > >
>> > > But why aren't the __rcu_sync_enter() and rcu_sync_exit() calls inside
>> > > the "if" statement?
>> >
>> > Yes, if we do __rcu_sync_enter() inside "if", then rcu_sync_exit() can't
>> > hit GP_ENTER.
>> >
>> > But why we should disallow this use-case? It does not complicate the code
>> > at all.
>>
>> I do agree that it doesn't complicate the current implementation.
>> But it relies on a global lock, so I am not at all confident that this
>> implementation is the final word.
>
> Hmm. which global lock? Or did you mean freeze_super(), not rcu_sync?
>
>> And speaking of global locks, failing to discourage the pattern above
>> means that the code is unnecessarily acquiring three global locks,
>> which doesn't seem like a good thing to me.
>
> Well, I do not agree, but this wasn't written by me. Just in case, all these
> locks above are not really global, they are per-sb, but this is minor.
>
> And the patches which changed sb->s_writers to use percpu_rw_semaphore/rcu_sync
> didn't change this logic.
>
> Except the old implementation was buggy, and the readers were slower than now.
>
>> I agree that there are use cases for beginning-of-time __rcu_sync_enter()
>> or whatever we end up naming it.
>
> OK, at least iiuc you agree that cgroup_init() can use __rcu_sync_enter().
> As for other potential use-cases, we will disccuss this later. I will have
> to CC you anyway ;)
>
> So I'll send v2 with renames after I test it. Thanks again.

Can you also make clear which patches of PeterZ's I should be adding
as well for testing?  I've lost the plot as to what goes with what..

thanks
-john