>
> 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"
Thanks
Zqiang
>
> Thanx, Paul
>
> >
> > Thanks
> > Zqiang
> >
> >
> >
> > Thanx, Paul
> >
> > >
> > > if (needwake && READ_ONCE(rtp->kthread_ptr))
> > > irq_work_queue(&rtpcp->rtp_irq_work);
> > > }
> > > @@ -681,7 +680,6 @@ static void __init
> > rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
> > > t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
> > > if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread,
> > OOM is now expected behavior\n", __func__, rtp->name))
> > > return;
> > > - smp_mb(); /* Ensure others see full kthread. */
> > > }
> > >
> > > #ifndef CONFIG_TINY_RCU
> > > --
> > > 2.17.1
> > >
> >
>
