Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve

[Peter Zijlstra]

On Fri, Jan 17, 2020 at 02:54:03PM +0800, Shaokun Zhang wrote:
> So how about this? ;-)
> 
>                 delta = prandom_u32_max(now - old);
> 
> +#ifdef CONFIG_UBSAN
>         /* Do not use atomic_add_return() as it makes UBSAN unhappy */
> +       old = (u32)atomic_read(p_id);
>         do {
> -               old = (u32)atomic_read(p_id);
>                 new = old + delta + segs;
> -       } while (atomic_cmpxchg(p_id, old, new) != old);
> +       } while (!atomic_try_cmpxchg(p_id, &old, new));
> +#else
> +       new = atomic_add_return(segs + delta, p_id);
> +#endif

That's crazy, just accept that UBSAN is taking bonghits and ignore it.
Use atomic_add_return() unconditionally.