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

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Date:   Fri, 18 Oct 2019 12:26:55 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Martin Lau <kafai@...com>
Cc:     "daniel\@iogearbox.net" <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        "bpf\@vger.kernel.org" <bpf@...r.kernel.org>,
        "netdev\@vger.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

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?

-Toke