On Thu, Sep 05, 2024 at 08:41:02PM +0200, Frederic Weisbecker wrote:
> Le Thu, Sep 05, 2024 at 08:32:16PM +0200, Frederic Weisbecker a écrit :
> > Le Wed, Sep 04, 2024 at 06:52:36AM -0700, Paul E. McKenney a écrit :
> > > > Yes, I'm preparing an update for the offending patch (which has one more
> > > > embarassing issue while I'm going through it again).
> > >
> > > Very good, thank you!
> >
> > So my proposal for a replacement patch is this (to replace the patch
> > of the same name in Neeraj tree):
>
> FYI, the diffstat against the previous version of the same patch is as
> follows.
> The rationale being:
>
> 1) rdp->nocb_cb_kthread doesn't need to be protected by nocb_gp_kthread_mutex
>
> 2) Once rcuoc is parked, we really _must_ observe the callback list counter
> decremented
> after the barrier's completion.
>
> 3) This fixes another issue: rcuoc must be parked _before_
> rcu_nocb_queue_toggle_rdp() is called, otherwise a nocb locked sequence
> within rcuoc would race with rcuog clearing SEGCBLIST_OFFLOADED
> concurrently,
> leaving the nocb locked forever.
Thank you!!!
Just to make sure that I understand, I apply this patch on top of
Neeraj's current set of branches to get the fix, correct?
Thanx, Paul
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 755ada098035..97b99cd06923 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1056,6 +1056,13 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
> /* Flush all callbacks from segcblist and bypass */
> rcu_barrier();
>
> + /*
> + * Make sure the rcuoc kthread isn't in the middle of a nocb locked
> + * sequence while offloading is deactivated, along with nocb locking.
> + */
> + if (rdp->nocb_cb_kthread)
> + kthread_park(rdp->nocb_cb_kthread);
> +
> rcu_nocb_lock_irqsave(rdp, flags);
> WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> @@ -1064,13 +1071,11 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data
> *rdp)
> wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
>
> mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
> +
> if (rdp_gp->nocb_gp_kthread) {
> if (wake_gp)
> wake_up_process(rdp_gp->nocb_gp_kthread);
>
> - if (rdp->nocb_cb_kthread)
> - kthread_park(rdp->nocb_cb_kthread);
> -
> swait_event_exclusive(rdp->nocb_state_wq,
> rcu_nocb_rdp_deoffload_wait_cond(rdp));
> } else {
