linux-kernel - Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading

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Date:   Wed, 29 Mar 2023 18:07:58 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent
 (de-)offloading

On Sun, Mar 26, 2023 at 02:45:18PM -0700, Paul E. McKenney wrote:
> On Sun, Mar 26, 2023 at 10:01:34PM +0200, Frederic Weisbecker wrote:
> > > > > >  	/* Snapshot count of all CPUs */
> > > > > >  	for_each_possible_cpu(cpu) {
> > > > > >  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > > > -		int _count = READ_ONCE(rdp->lazy_len);
> > > > > > +		int _count;
> > > > > > +
> > > > > > +		if (!rcu_rdp_is_offloaded(rdp))
> > > > > > +			continue;
> > > > > 
> > > > > If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero?
> > > > > 
> > > > > Or can it contain garbage after a de-offloading operation?
> > > > 
> > > > If it's deoffloaded, ->lazy_len is indeed (supposed to be) guaranteed to be zero.
> > > > Bypass is flushed and disabled atomically early on de-offloading and the
> > > > flush resets ->lazy_len.
> > > 
> > > Whew!  At the moment, I don't feel strongly about whether or not
> > > the following code should (1) read the value, (2) warn on non-zero,
> > > (3) assume zero without reading, or (4) some other option that is not
> > > occurring to me.  Your choice!
> > 
> > (2) looks like a good idea!
> 
> Sounds good to me!

So since we now iterate rcu_nocb_mask after the patchset, there is no more
deoffloaded rdp to check. Meanwhile I put a WARN in the new series making
sure that an rdp in rcu_nocb_mask is also offloaded (heh!)