netdev - Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read

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Message-ID: <0c5f827f-1b39-c850-f450-171c1a6e92b9@gmail.com>
Date:   Thu, 9 Jan 2020 15:34:10 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     John Fastabend <john.fastabend@...il.com>, bjorn.topel@...il.com,
        bpf@...r.kernel.org, toke@...hat.com
Cc:     netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net
Subject: Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required
 rcu_read_{un}lock()

On 2020/01/09 15:01, Toshiaki Makita wrote:
> On 2020/01/09 6:35, John Fastabend wrote:
>> Now that we depend on rcu_call() and synchronize_rcu() to also wait
>> for preempt_disabled region to complete the rcu read critical section
>> in __dev_map_flush() is no longer relevant.
>>
>> These originally ensured the map reference was safe while a map was
>> also being free'd. But flush by new rules can only be called from
>> preempt-disabled NAPI context. The synchronize_rcu from the map free
>> path and the rcu_call from the delete path will ensure the reference
>> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
>> pair to avoid any confusion around how this is being protected.
>>
>> If the rcu_read_lock was required it would mean errors in the above
>> logic and the original patch would also be wrong.
>>
>> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
>> Signed-off-by: John Fastabend <john.fastabend@...il.com>
>> ---
>>   kernel/bpf/devmap.c |    2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>> index f0bf525..0129d4a 100644
>> --- a/kernel/bpf/devmap.c
>> +++ b/kernel/bpf/devmap.c
>> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
>>       struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
>>       struct xdp_bulk_queue *bq, *tmp;
>> -    rcu_read_lock();
>>       list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
>>           bq_xmit_all(bq, XDP_XMIT_FLUSH);
>> -    rcu_read_unlock();
> 
> I introduced this lock because some drivers have assumption that
> .ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
> 
> Maybe devmap deletion logic does not need this anymore, but is it
> OK to drivers?

More references for the driver problem:
- Lockdep splat in virtio_net: https://lists.openwall.net/netdev/2019/04/24/282
- Discussion for fix: https://lists.openwall.net/netdev/2019/04/25/234

Toshiaki Makita