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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 17 Feb 2020 08:39:45 +0100
Message-ID: <CAHmME9q+YYia0H3upW7ikwSii_XegNNSBkVxP-1mxaHyEVmBxA@mail.gmail.com>
Subject: Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     eric.dumazet@gmail.com, cai@lca.pw, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
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On 2/17/20, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> Jason A. Donenfeld <Jason@zx2c4.com> 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.