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

Eric Dumazet
2020-02-06T17:10:38Z

On 2/6/20 8:38 AM, Jason A. Donenfeld 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?
>
> Jason
>

Unfortunately we do not have ADD_ONCE() or something like that.

Sure, on x86 we could get much better code generation.

If we agree a READ_ONCE() was needed at the read side,
then a WRITE_ONCE() is needed as well on write sides.

If we believe load-tearing and/or write-tearing must not ever happen,
then we must document this.