On Fri, Dec 02, 2022 at 11:35:09PM +0000, Zhang, Qiang1 wrote:
> On Wed, Nov 30, 2022 at 01:12:53PM +0800, Zqiang wrote:
> > Assume that the current RCU-task belongs to per-CPU callback queuing
> > mode and the rcu_task_cb_adjust is true.
> >
> > CPU0 CPU1
> >
> > rcu_tasks_need_gpcb()
> > ncbsnz == 0 and
> > ncbs < rcu_task_collapse_lim
> >
> > invoke call_rcu_tasks_generic()
> > enqueue callback to CPU1
> > (CPU1 n_cbs not equal zero)
> >
> > if (rcu_task_cb_adjust &&
> > ncbs <= rcu_task_collapse_lim)
> > if (rtp->percpu_enqueue_lim > 1)
> > rtp->percpu_enqueue_lim = 1;
> > rtp->percpu_dequeue_gpseq =
> > get_state_synchronize_rcu();
> >
> >
> > A full RCU grace period has passed
> >
> >
> >I don't see how this grace period can elapse. The rcu_tasks_need_gpcb()
> >function is invoked only from rcu_tasks_one_gp(), and while holding
> >->tasks_gp_mutex.
>
>
> Hi Paul
>
> I mean that It's the RCU grace period instead of the RCU task grace period.
>
> rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
>
> get_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
>
> There is a window period between these two calls, preemption may occur.
> A full RCU grace period may have passed.
> when we run it again we find get_state_synchronize_rcu() == true.
>
>Ah, thank you!
>
>But for that issue, why not just place both "if" statements into an
>RCU read-side critical section?
Ah, a better method(my brain didn't think of this way at the first time), I have been send v2.
Thanks
Zqiang
That would maintain the diagnostics,
>just in case a bug was introduced into the shrinking process.
>
> Thanx, Paul
>
> Thanks
> Zqiang
>
>
> >
> >What am I missing here?
> >
> > Thanx, Paul
>
> > if (rcu_task_cb_adjust && !ncbsnz &&
> > poll_state_synchronize_rcu(
> > rtp->percpu_dequeue_gpseq) == true
> > if (rtp->percpu_enqueue_lim <
> > rtp->percpu_dequeue_lim)
> > rtp->percpu_dequeue_lim = 1
> > for (cpu = rtp->percpu_dequeue_lim;
> > cpu < nr_cpu_ids; cpu++)
> > find CPU1 n_cbs is not zero
> > trigger warning
> >
> > The above scenario will not only trigger WARN_ONCE(), but also set the
> > rcu_tasks structure's->percpu_dequeue_lim is one when CPU1 still have
> > callbacks, which will cause the callback of CPU1 to have no chance to be
> > called.
> >
> > This commit add per-cpu callback check(except CPU0) before set the rcu_tasks
> > structure's->percpu_dequeue_lim to one, if other CPUs(except CPU0) still have
> > callback, not set the rcu_tasks structure's->percpu_dequeue_lim to one, set it
> > until the all CPUs(except CPU0) has no callback.
> >
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > ---
> > kernel/rcu/tasks.h | 13 ++++++++-----
> > 1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index e4f7d08bde64..690af479074f 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -433,14 +433,17 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
> > poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq)) {
> > raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
> > if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
> > + for (cpu = rtp->percpu_enqueue_lim; cpu < nr_cpu_ids; cpu++) {
> > + struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> > +
> > + if(rcu_segcblist_n_cbs(&rtpcp->cblist)) {
> > + raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
> > + return needgpcb;
> > + }
> > + }
> > WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
> > pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
> > }
> > - for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
> > - struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
> > -
> > - WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
> > - }
> > raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
> > }
> >
> > --
> > 2.25.1
> >
