Re: [PATCH RFC v3 03/13] rcu-tasks: Add a Tasks RCU implementation for reader-marked trampolines

[Josef Bacik <josef@toxicpanda.com>]

On Thu, 17 Sep 2026 22:20:17 +0200, Frederic Weisbecker wrote:
> > +	lockdep_assert_irqs_disabled();
> > +	WRITE_ONCE(t->rcu_tasks_irq_ip, ip);
> > +	t->rcu_tasks_exit_cpu = smp_processor_id();
> > +	raw_spin_lock_rcu_node(rtpcp);
> > +	list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list);
> > +	raw_spin_unlock_rcu_node(rtpcp);
>
> I don't think we can do that. This is too much unconditional overhead
> on the hot preemption path. rcu_tasks_trampoline_text() should be
> a condition here.

Sorry, I missed this one before sending v4/v5. Agreed, and it is gone
for v6: the hook is now just the WRITE_ONCE() of the IP plus the
rcu_tasks_trampoline_text() check, and the exit side a single store.

The IP store itself has to stay unconditional because of the kprobe
jump optimizer: its window is ordinary text, so a task parked there
before the optimizer decided to patch was not "trampoline text" when it
was preempted, and the optimizer needs to find it afterwards.

> And do we really need to maintain both lists? I understand that they
> have different purposes.
[...]
> Can the latter replace the former?

With the above there is only the holdout list left. The optimizer's
rcu_tasks_wait_irq_preempted() now does what classic does for its scan:
walk the task list plus the per-CPU exit lists (exit_tasks_rcu_start()
and friends stay shared between the two flavors for that), checking each
task's recorded IP. That is a slow path that only kprobe optimization
hits.

And thanks for picking up the core-RCU follow-on.

Josef