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

Herbert Xu
2020-02-17T03:25:08Z

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?

Thanks,
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
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