Re: [PATCH] rcu-tasks: Make shrink down to a single callback queue safely

[Paul E. McKenney]

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?  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
> >