On Wed, Mar 10, 2021 at 11:05:07PM +0100, Frederic Weisbecker wrote:
> On Tue, Mar 02, 2021 at 05:15:57PM -0800, Paul E. McKenney wrote:
> > The first question is of course: Did you try this with lockdep enabled? ;-)
>
> Yep I always do. But I may miss some configs on my testings. I usually
> test at least TREE01 on x86 and arm64.
>
> > > @@ -1702,43 +1692,50 @@ bool rcu_is_nocb_cpu(int cpu)
> > > return false;
> > > }
> > >
> > > -/*
> > > - * Kick the GP kthread for this NOCB group. Caller holds ->nocb_lock
> > > - * and this function releases it.
> > > - */
> > > -static bool wake_nocb_gp(struct rcu_data *rdp, bool force,
> > > - unsigned long flags)
> > > - __releases(rdp->nocb_lock)
> > > +static bool __wake_nocb_gp(struct rcu_data *rdp_gp,
> > > + struct rcu_data *rdp,
> > > + bool force, unsigned long flags)
> > > + __releases(rdp_gp->nocb_gp_lock)
> > > {
> > > bool needwake = false;
> > > - struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
> > >
> > > - lockdep_assert_held(&rdp->nocb_lock);
> > > if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
> > > - rcu_nocb_unlock_irqrestore(rdp, flags);
> > > + raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
> > > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> > > TPS("AlreadyAwake"));
> > > return false;
> > > }
> > >
> > > - if (READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT) {
> > > - WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
> > > - del_timer(&rdp->nocb_timer);
> > > + if (rdp_gp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
> >
> > So there are no longer any data races involving ->nocb_defer_wakeup?
> >
> > (Yes, I could fire up KCSAN, but my KCSAN-capable system is otherwise
> > occupied for several more hours.)
>
> To be more specific, there is no more unlocked write to the timer
> (queue/cancel)
> and its nocb_defer_wakeup matching state. And there is only one (on purpose)
> racy
> reader of ->nocb_defer_wakeup which is the non-timer deferred wakeup.
>
> So the writes to the timer keep their WRITE_ONCE() and only the reader in
> do_nocb_deferred_wakeup() keeps its READ_ONCE(). Other readers are protected
> by the ->nocb_gp_lock.
>
> > > +
> > > // Advance callbacks if helpful and low contention.
> > > needwake_gp = false;
> > > if (!rcu_segcblist_restempty(&rdp->cblist,
> > > @@ -2178,11 +2182,18 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> > > my_rdp->nocb_gp_bypass = bypass;
> > > my_rdp->nocb_gp_gp = needwait_gp;
> > > my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
> > > - if (bypass && !rcu_nocb_poll) {
> > > - // At least one child with non-empty ->nocb_bypass, so set
> > > - // timer in order to avoid stranding its callbacks.
> > > + if (bypass) {
> > > raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
> > > - mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
> > > + // Avoid race with first bypass CB.
> > > + if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
> > > + WRITE_ONCE(my_rdp->nocb_defer_wakeup,
> > > RCU_NOCB_WAKE_NOT);
> > > + del_timer(&my_rdp->nocb_timer);
> > > + }
> >
> > Given that the timer does not get queued if rcu_nocb_poll, why not move the
> > above "if" statement under the one following?
>
> It's done later in the set.
>
> >
> > > + if (!rcu_nocb_poll) {
> > > + // At least one child with non-empty ->nocb_bypass, so
> > > set
> > > + // timer in order to avoid stranding its callbacks.
> > > + mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
> > > + }
> > > raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
> > > }
> > > if (rcu_nocb_poll) {
> > > @@ -2385,7 +2399,10 @@ static void do_nocb_deferred_wakeup_timer(struct
> > > timer_list *t)
> > > */
> > > static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
> > > {
> > > - if (rcu_nocb_need_deferred_wakeup(rdp))
> > > + if (!rdp->nocb_gp_rdp)
> > > + return false;
> >
> > This check was not necessary previously because each CPU used its own rdp,
> > correct?
>
> Exactly!
>
> > The theory is that this early return is taken only during boot,
> > and that the spawning of the kthreads will act as an implicit wakeup?
>
> You guessed right! That probably deserve a comment.
OK, I have queued these for for further review and testing. Also to
look at the overall effect. Thank you very much!
Thanx, Paul
