Re: [PATCH v7 6/6] rcu/segcblist: Add additional comments to explain smp_mb()

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Oct 19, 2020 at 5:37 AM Frederic Weisbecker <frederic@kernel.org> wrote:
>
[..]
> > >
> > > I'm very likely missing something obvious somewhere.
> > >
> > >       CPU 0                                CPU 1
> > >       rcu_barrier()                        call_rcu()/rcu_segcblist_enqueue()
> > >       ------------                         --------
> > >
> > >                                            smp_mb();
> > >                                            inc_len();
> > >                                        smp_mb();
> > >                                        queue callback;
> > >       for_each_possible_cpu(cpu)
> > >           if (!rcu_segcblist_n_cbs(&rdp->cblist))
> > >           continue;
> > >
> >
> > >                                             invoke_callback
> >
> > If CPU 0 saw the enqueue of the callback (that is the CPU 1's writes to the
> > segcb_list propagated to CPU 0), then it would have also seen the
> > effects of the inc_len. I forced this case in my last litmus test by this
> > code in P1():
>
> But then I can't find to which part of rcu_barrier() this refers to.
> I see the len read before anything else.
>
> >
> >         r1 = READ_ONCE(*enq);
> >         smp_mb();               /* barrier Just for test purpose to show that the.. */
> >                                 /* ..rcu_barrier() saw list modification */
> >
> > On the other hand, if CPU 0 did not see the enqueue, then there is really no
> > issue. Since that is the same case where call_rcu() happened _after_ the
> > rcu_barrier() and there's no race. rcu_barrier() does not need to wait if
> > there was no callback enqueued.
> >
> > This is not exactly the easiest thing to explain, hence the litmus.
>
> Now, reading the documentation of rcu_barrier() (thanks to you!):
>
>     Pseudo-code using rcu_barrier() is as follows:
>
>    1. Prevent any new RCU callbacks from being posted.
>    2. Execute rcu_barrier().
>    3. Allow the module to be unloaded.
>

Basically, you are saying that if all CPUs agree that len == 0
henceforth (through other memory barriers), then callback enqueuing
does not need a memory barrier before setting length to 0.

I think that makes sense but is it worth removing the memory barrier
before WRITE(len, 1) and hoping after #1, the caller would have
ensured things are fine? Also I am not sure if the above is the only
usecase for rcu_barrier().

> I think with point 1, it is assumed that the caller of rcu_barrier() must have
> not only stopped but also sync'ed with the possible enqueuers. Correct me if I'm wrong
> here. So for example if a kthread used to post the module RCU callbacks, calling kthread_stop()
> does the job as it prevents from further RCU callbacks from being enqueued and it also syncs
> with the kthread thanks to the completion implied by any caller of kthread_stop() which then
> sees what the kthread has read and written, including RCU callbacks enqueued. So if the caller
> of kthread_stop() calls rcu_barrier() right after, rcu_barrier() should see at least the len
> corresponding to the last enqueue.
>
> cancel_work_sync() also seem to really sync as well. I'm less sure about del_timer_sync().
>
> Say we have:
>
> expire_timers (CPU 0)                               CPU 1
> -------------                                       -----------
> detach_timer(timer)
> raw_spin_unlock(&base->lock);
> call_timer_fn(timer, fn, baseclk);
>    -> enqueue callback
> //would need at least smp_wmb() here
> base->running_timer = NULL;
>
>                                                     del_timer_sync() {
>                                                         raw_spin_lock(&base->lock);
>                                                         if (base->running_timer != timer)
>                                                             ret = detach_if_pending(timer, base, true);
>                                                                 if (!timer_pending())
>                                                                     return 0;
>                                                         raw_spin_unlock(&base->lock);
>                                                     }
>                                                     //would need at least smp_rmb() here

Regarding "would need at least smp_rmb.." :
But the rcu_barrier() has the control dependency we discussed in last
emails, between READ(len) and whatever follows the rcu_barrier().
That itself will provide the ordering right?

>                                                     //although rcu_seq_start() implies a full barrier
>                                                     rcu_barrier() {
>                                                         // Sees rcu_segcblist_n_cbs(rdp(CPU 0)->cblist) == 0
>                                                         // So ignore it
>
>
> But I'm sure I'm missing something obvious. That's my specialism.

I could be missing something too :-/. But I'll include this patch in
my next posting anyway and let us also maybe see if Paul disagrees.

thanks,

 - Joel