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Date:   Fri, 17 Jan 2020 13:32:53 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Shaokun Zhang <zhangshaokun@hisilicon.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, jinyuqi@huawei.com,
        kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org, edumazet@google.com,
        guoyang2@huawei.com, Will Deacon <will@kernel.org>
Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve
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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.