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

Marco Elver
2020-02-07T10:35:34Z

On Thu, 6 Feb 2020 at 18:10, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
>
> 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.

Just FYI, forgot to mention: Recent KCSAN by default will forgive
unannotated aligned writes up to word-size, making the assumptions
these are safe. This would include things like 'var++' if there is
only a single writer. This was added because of kernel-wide
preferences we were told about.

Since I cannot verify if this assumption is always correct, I would
still prefer to mark writes if at all possible. In the end it's up to
maintainers.

Thanks,
-- Marco