netdev - Re: [PATCH 1/2] net: dsa: RCU-protect dsa_ptr in struct net

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Message-ID: <c7a52a818c1ae49ad7e44bb82fcea53d7f53d6e0.camel@siemens.com>
Date: Tue, 17 Sep 2024 08:25:19 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "gur.stavi@...wei.com" <gur.stavi@...wei.com>
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Subject: Re: [PATCH 1/2] net: dsa: RCU-protect dsa_ptr in struct net_device

Hello Gur!

On Tue, 2024-09-17 at 11:10 +0300, Gur Stavi wrote:
> > @@ -1594,10 +1592,11 @@ void dsa_switch_shutdown(struct dsa_switch *ds)
> >   	}
> > 
> >   	/* Disconnect from further netdevice notifiers on the conduit,
> > -	 * since netdev_uses_dsa() will now return false.
> > +	 * from now on, netdev_uses_dsa_currently() will return false.
> >   	 */
> >   	dsa_switch_for_each_cpu_port(dp, ds)
> > -		dp->conduit->dsa_ptr = NULL;
> > +		rcu_assign_pointer(dp->conduit->dsa_ptr, NULL);
> > +	synchronize_rcu();
> > 
> >   	rtnl_unlock();
> >   out:
> 
> Hi, I am a newbie here. Thanks for the opportunity for learning more
> about rcu.
> Wouldn't it make more sense to call synchronize_rcu after rtnl_unlock?

This is indeed a question which is usually resolved other way around
(making locked section shorter), but in this particular case I thought that:
- we actually don't need giving rtnl lock sooner, which would potentially
  make synchronize_rcu() call longer (if another thread manages to wake up
  and claim the rtnl lock before synchronize_rcu())
- we are in shutdown phase, we don't need to minimize lock contention, we
  need to minimize the overall shutdown time

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com