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

[Jason A. Donenfeld]

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