netdev - Re: [PATCH bpf v2] xdp: Handle device unregister for devmap

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Date:   Fri, 18 Oct 2019 16:50:57 +0000
From:   Martin Lau <kafai@...com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     "daniel@...earbox.net" <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: Re: [PATCH bpf v2] xdp: Handle device unregister for devmap_hash map
 type

On Fri, Oct 18, 2019 at 12:26:55PM +0200, Toke Høiland-Jørgensen wrote:
> Martin Lau <kafai@...com> writes:
> 
> > On Thu, Oct 17, 2019 at 12:52:32PM +0200, Toke Høiland-Jørgensen wrote:
> >> It seems I forgot to add handling of devmap_hash type maps to the device
> >> unregister hook for devmaps. This omission causes devices to not be
> >> properly released, which causes hangs.
> >> 
> >> Fix this by adding the missing handler.
> >> 
> >> Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
> >> Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> >> ---
> >> v2:
> >>   - Grab the update lock while walking the map and removing entries.
> >> 
> >>  kernel/bpf/devmap.c | 37 +++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 37 insertions(+)
> >> 
> >> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> >> index d27f3b60ff6d..a0a1153da5ae 100644
> >> --- a/kernel/bpf/devmap.c
> >> +++ b/kernel/bpf/devmap.c
> >> @@ -719,6 +719,38 @@ const struct bpf_map_ops dev_map_hash_ops = {
> >>  	.map_check_btf = map_check_no_btf,
> >>  };
> >>  
> >> +static void dev_map_hash_remove_netdev(struct bpf_dtab *dtab,
> >> +				       struct net_device *netdev)
> >> +{
> >> +	unsigned long flags;
> >> +	int i;
> > dtab->n_buckets is u32.
> 
> Oh, right, will fix.
> 
> >> +
> >> +	spin_lock_irqsave(&dtab->index_lock, flags);
> >> +	for (i = 0; i < dtab->n_buckets; i++) {
> >> +		struct bpf_dtab_netdev *dev, *odev;
> >> +		struct hlist_head *head;
> >> +
> >> +		head = dev_map_index_hash(dtab, i);
> >> +		dev = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),
> > The spinlock has already been held.  Is rcu_deref still needed?
> 
> I guess it's not strictly needed, but since it's an rcu-protected list,
> and hlist_first_rcu() returns an __rcu-annotated type, I think we will
> get a 'sparse' warning if it's omitted, no?
> 
> And since it's just a READ_ONCE, it doesn't actually hurt since this is
> not the fast path, so I'd lean towards just keeping it? WDYT?
>
Can hlist_for_each_safe() be used instead then?
A bonus is the following long line will go away.
I think the change will be simpler also.


> +                                    struct bpf_dtab_netdev,
> +                                    index_hlist);
> +
> +             while (dev) {
> +                     odev = (netdev == dev->dev) ? dev : NULL;
> +                     dev = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(&dev->index_hlist)),
> +                                            struct bpf_dtab_netdev,
> +                                            index_hlist);
> +
> +                     if (odev) {
> +                             hlist_del_rcu(&odev->index_hlist);
> +                             call_rcu(&odev->rcu,
> +                                      __dev_map_entry_free);
> +                     }
> +             }
> +     }
> +     spin_unlock_irqrestore(&dtab->index_lock, flags);
> +}
> +