linux-kernel - Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

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Message-ID: <20211213112246.GA782195@lothringen>
Date:   Mon, 13 Dec 2021 12:22:46 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc:     David Woodhouse <dwmw2@...radead.org>, paulmck@...nel.org,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, urezki@...il.com,
        boqun.feng@...il.com
Subject: Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

On Mon, Dec 13, 2021 at 02:25:30PM +0530, Neeraj Upadhyay wrote:
> Hi David,
> 
> Thanks for the review; some replies inline.
> 
> On 12/13/2021 1:48 PM, David Woodhouse wrote:
> > On Sat, 2021-12-11 at 22:31 +0530, Neeraj Upadhyay wrote:
> > > When multiple CPUs in the same nocb gp/cb group concurrently
> > > come online, they might try to concurrently create the same
> > > rcuog kthread. Fix this by using nocb gp CPU's spawn mutex to
> > > provide mutual exclusion for the rcuog kthread creation code.
> > > 
> > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> > > ---
> > > Change in v2:
> > >   Fix missing mutex_unlock in nocb gp kthread creation err path.
> > 
> > I think this ends up being not strictly necessary in the short term too
> > because we aren't currently planning to run rcutree_prepare_cpu()
> > concurrently anyway. But harmless and worth fixing in the longer term.
> > 
> > Although, if I've already added a mutex for adding the boost thread,
> > could we manage to use the *same* mutex instead of adding another one?
> > 
> 
> Let me think about it; the nocb-gp and nocb-cb kthreads are grouped based on
> rcu_nocb_gp_stride; whereas, boost kthreads are per rnp. So, I need to see
> how we can use a common mutex for both.
> 
> 
> > Acked-by: David Woodhouse <dwmw@...zon.co.uk>
> > +                       mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> > >                          return;
> > > +               }
> > >                  WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
> > >          }
> > > +        mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> > > 
> > >          /* Spawn the kthread for this CPU. */
> > 
> > Some whitespace damage there.
> 
> Will fix in next version.

I was about to ack the patch but, should we really add code that isn't going to
be necessary before a long while?

Thanks!


> 
> Thanks
> Neeraj
> 
> >