Subject: Re: [PATCH bpf v2] xdp: Handle device unregister for devmap_hash map type

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 18 Oct 2019 21:28:14 +0200

Martin Lau <[email-masked]> writes:

> On Fri, Oct 18, 2019 at 12:26:55PM +0200, Toke Høiland-Jørgensen wrote:
>> Martin Lau <[email-masked]> 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 <[email-masked]>
>> >> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> >> ---
>> >> 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.

Ohhh, yes it can! I was looking for that variant of the for_each macro
(the removal-safe one) and scratching my head as to why it wasn't there.
Dunno how I missed that; thanks, will fix and resend! :)

-Toke