Re: KCSAN: data-race in __alloc_file / __alloc_file

[Eric Dumazet]

On Mon, Nov 11, 2019 at 12:47 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Nov 11, 2019 at 12:43 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Yeah, maybe we could have some model for marking "this is statistics,
> > doesn't need to be exact".
>
> Side note: that marking MUST NOT be "READ_ONCE + WRITE_ONCE", because
> that makes gcc create horrible code, and only makes the race worse.
>
> At least with a regular add, it might stay as a single r-m-w
> instruction on architectures that have that, and makes the quality of
> the statistics slightly better (no preemption etc).
>
> So that's an excellent example of where changing code to use
> WRITE_ONCE actually makes the code objectively worse in practice -
> even if it might be the same in theory.

Yes, I believe that was the rationale of the ADD_ONCE() thing I
mentioned earlier.

I do not believe we have a solution right now ?

We have similar non atomic increments in some virtual network drivers
doing "dev->stats.tx_errors++;"  in their error path.