Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()

[Jason A. Donenfeld]

On 2/17/20, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>> Hi Eric,
>>
>> On Tue, Feb 04, 2020 at 01:40:29PM -0500, Qian Cai wrote:
>>> -     list->qlen--;
>>> +     WRITE_ONCE(list->qlen, list->qlen - 1);
>>
>> Sorry I'm a bit late to the party here, but this immediately jumped out.
>> This generates worse code with a bigger race in some sense:
>>
>> list->qlen-- is:
>>
>>   0:   83 6f 10 01             subl   $0x1,0x10(%rdi)
>>
>> whereas WRITE_ONCE(list->qlen, list->qlen - 1) is:
>>
>>   0:   8b 47 10                mov    0x10(%rdi),%eax
>>   3:   83 e8 01                sub    $0x1,%eax
>>   6:   89 47 10                mov    %eax,0x10(%rdi)
>>
>> Are you sure that's what we want?
>
> Fixing these KCSAN warnings is actively making the kernel worse.
>
> Why are we still doing this?
>
Not necessarily a big fan of this either, but just for the record here
in case it helps, while you might complain about instruction size
blowing up a bit, cycle-wise these wind up being about the same
anyway. On x86, one instruction != one cycle.