On Sat, Jul 11, 2026 at 05:09:22AM +0000, Zqiang wrote:
> >
> > On Fri, Jul 10, 2026 at 11:18:05PM +0000, Zqiang wrote:
> >
> > >
> > > On Fri, Jul 10, 2026 at 07:50:28PM +0800, Zqiang wrote:
> > >
> > > >
> > > > The smp_mb() after kthread_run() in rcu_spawn_tasks_kthread_generic()
> > > > from early 'commit eacd6f04a133 ("rcu-tasks: Move Tasks RCU to its
> > > > own file")' and 'commit 84a8f446ffd7 ("rcu: Defer rcu_tasks_kthread()
> > > > creation till first call_rcu_tasks()")', the pairing as follows:
> > > >
> > > > rcu_spawn_tasks_kthread()
> > > > ->t = kthread_run(rcu_tasks_kthread, ...);
> > > > ->smp_mb(); /* Ensure others see full kthread. */
> > > > ->WRITE_ONCE(rcu_tasks_kthread_ptr, t);
> > > >
> > > > call_rcu_tasks()
> > > > ->if (READ_ONCE(rcu_tasks_kthread_ptr))
> > > > ->wake_up(&rcu_tasks_cbs_wq)
> > > > ->try_to_wake_up()
> > > > lock pi_lock
> > > > ->smp_mb__after_spinlock()
> > > > //see full kthread
> > > >
> > > > Currently, the 'commit d119357d0743 ("rcu-tasks: Treat only synchronous
> > > > grace periods urgently")' moved kthread_ptr assignment into the
> > > > rcu_tasks_kthread(), the following pairings are sufficient:
> > > >
> > > > The rq->lock/unlock from wake_up_process() in kthread_run() and
> > > > __schedule() provides a memory barrier when the kthread is first
> > > > scheduled, this ensures the kthread itself observes all of the
> > > > kthread's initialization.
> > > >
> > > > The kthread's smp_store_release(&rtp->kthread_ptr, ...) in
> > > > rcu_tasks_kthread() and smp_load_acquire(&rtp->kthread_ptr) in
> > > > call_rcu_tasks_generic() compose release/acquire pairing, the
> > > > cumulativity of smp_store_release() propagates visibility of the
> > > > kthread's initialization through the scheduler chain.
> > > >
> > > > This commit therefore remove smp_mb() in
> > > rcu_spawn_tasks_kthread_generic().
> > > >
> > > > Signed-off-by: Zqiang <[email protected]>
> > > >
> > > Very good, a removal of an smp_mb() that also simplifies analysis!
> > >
> > > One question...
> > >
> > > >
> > > > ---
> > > > kernel/rcu/tasks.h | 2 --
> > > > 1 file changed, 2 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > > index 5a8db89135eb..18ab335665e3 100644
> > > > --- a/kernel/rcu/tasks.h
> > > > +++ b/kernel/rcu/tasks.h
> > > > @@ -395,7 +395,6 @@ static void call_rcu_tasks_generic(struct rcu_head
> > > *rhp, rcu_callback_t func,
> > > > raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
> > > > }
> > > > rcu_read_unlock();
> > > > - /* We can't create the thread unless interrupts are enabled. */
> > > >
> > > Why remove this comment? People reading this code might wonder why we
> > > are not just invoking rcu_spawn_tasks_kthread_generic() to create the
> > > task here.
> > >
> > > Now, if you say that the current comment is confusing because if that
> > > kthread already exists, we also defer the wakeup, fair enough. But in
> > > that case, could you please propose a replacement comment, perhaps as
> > > a separate patch?
> > >
> > > How about the following comment:
> > >
> > > /* Use irq_work do deferred wakeup to avoid potential lockdep splat */
> > >
> > > If directly use rcuwait_wake_up() to wakup, the lockdep will possible
> > > spalt，
> > > (for example we hook trace_sched_wakeup() tracepoint, and in our hook
> > > func inovke call_rcu_tasks())
> > >
> > Fair enough. But the reason for the READ_ONCE(rtp->kthread_ptr) part of
> > the condition of that "if" statement is that we cannot create a kthread
> > with interrupts disabled, either because it is too early in boot or
> > because one of the scheduler locks might be held.
> >
> > Maybe that is obvious? But we did add that comment at some point.
> >
> > Thoughts?
>
> Ok, I find that:
>
> c63eb17ff06 ("rcu: Create call_rcu_tasks() kthread at boot time")
> 4929c913bda ("rcu: Make call_rcu_tasks() tolerate first call with irqs
> disabled")
>
> As a separate patch send, the update comments are as follows:
>
> "We can't create the kthread with interrupts disabled,
> or a sinlock might be held, create the kthread moved
> at core_initcall() time, at the same time, use the irq_work
> does deferred wakeup to avoid potential lockdep splat"
Very good, looking forward to the updates!
Thanx, Paul
