netdev - Re: [PATCH net 1/2] net: disable netpoll on fresh napis

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Message-ID: <20200827081003.289009f4@kicinski-fedora-PC1C0HJN>
Date:   Thu, 27 Aug 2020 08:10:03 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, michael.chan@...adcom.com,
        netdev@...r.kernel.org, kernel-team@...com,
        Rob Sherwood <rsher@...com>
Subject: Re: [PATCH net 1/2] net: disable netpoll on fresh napis

On Thu, 27 Aug 2020 00:25:31 -0700 Eric Dumazet wrote:
> On 8/26/20 12:40 PM, Jakub Kicinski wrote:
> > To ensure memory ordering is correct we need to use RCU accessors.
>
> > +	set_bit(NAPI_STATE_NPSVC, &napi->state);
> > +	list_add_rcu(&napi->dev_list, &dev->napi_list);
> 
> >  
> > -	list_for_each_entry(napi, &dev->napi_list, dev_list) {
> > +	list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) {
> >  		if (cmpxchg(&napi->poll_owner, -1, cpu) == -1) {
> >  			poll_one_napi(napi);
> >  			smp_store_release(&napi->poll_owner, -1);
> >   
> 
> You added rcu in this patch (without anything in the changelog).

I mentioned I need it for the barriers, in particular I wanted the
store release barrier in list_add. Not extremely clean :(

> netpoll_poll_dev() uses rcu_dereference_bh(), suggesting you might
> need list_for_each_entry_rcu_bh()

I thought the RCU flavors are mostly meaningless at this point,
list_for_each_entry_rcu() checks rcu_read_lock_any_held(). I can add
the definition of list_for_each_entry_rcu_bh() (since it doesn't exist)
or go back to non-RCU iteration (since the use is just documentation,
the code is identical). Or fix it some other way?