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

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Date:   Thu, 27 Aug 2020 10:47:53 -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 08:43:22 -0700 Eric Dumazet wrote:
> On 8/27/20 8:10 AM, Jakub Kicinski wrote:
> > 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 :(  
> 
> Hmmm, we also have smp_mb__after_atomic()

Pairing with the cmpxchg() on the netpoll side? Can do, I wasn't 
sure if the list operations themselves need some special care 
(like READ_ONCE/WRITE_ONCE)..

> >> 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?
> >   
> 
> Oh, I really thought list_for_each_entry_rcu() was only checking standard rcu.
> 
> I might have been confused because we do have hlist_for_each_entry_rcu_bh() helper.
> 
> Anyway, when looking at the patch I was not at ease because we do not have proper
> rcu grace period when a napi is removed from dev->napi_list. A driver might
> free the napi struct right after calling netif_napi_del()

Ugh, you're right. I didn't look closely enough at netif_napi_del():

	if (napi_hash_del(napi))
		synchronize_net();
	list_del_init(&napi->dev_list);

Looks like I can reorder these.. and perhaps make all dev->napi_list
accesses RCU, for netpoll?