linux-kernel - Re: [PATCH] rcutorture: Traverse possible cpu to set maxcpu in rcu_nocb

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Date:   Fri, 25 Aug 2023 10:28:37 +0800
From:   Z qiang <qiang.zhang1211@...il.com>
To:     paulmck@...nel.org
Cc:     joel@...lfernandes.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Traverse possible cpu to set maxcpu in rcu_nocb_toggle()

>
> On Thu, Aug 24, 2023 at 04:42:06PM +0800, Zqiang wrote:
> > Currently, the maxcpu is set by traversing online CPUs, however, if
> > the rcutorture.onoff_holdoff is set zero and onoff_interval is set
> > non-zero, and the some CPUs with larger cpuid has been offline before
> > setting maxcpu, for these CPUs, even if they are online again, also
> > cannot be offload or deoffload.
> >
> > This commit therefore use for_each_possible_cpu() instead of
> > for_each_online_cpu() in rcu_nocb_toggle().
> >
> > Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
> > ---
> >  kernel/rcu/rcutorture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index a58372bdf0c1..b75d0fe558ce 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -2131,7 +2131,7 @@ static int rcu_nocb_toggle(void *arg)
> >       VERBOSE_TOROUT_STRING("rcu_nocb_toggle task started");
> >       while (!rcu_inkernel_boot_has_ended())
> >               schedule_timeout_interruptible(HZ / 10);
> > -     for_each_online_cpu(cpu)
> > +     for_each_possible_cpu(cpu)
>
> Last I checked, bad things could happen if the code attempted to
> nocb_toggle a CPU that had not yet come online.  Has that changed?

For example, there are 8 online CPUs in the system, before we traversing online
CPUs and set maxcpu,  CPU7 has been offline, this causes us to miss nocb_toggle
for CPU7(maxcpu=6)

Even though we still use for_each_online_cpu(), the things described
above also happen.  before we toggle the CPU, this CPU has been offline.


Thanks
Zqiang


>
>                                                         Thanx, Paul
>
> >               maxcpu = cpu;
> >       WARN_ON(maxcpu < 0);
> >       if (toggle_interval > ULONG_MAX)
> > --
> > 2.17.1
> >