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Date:   Thu, 16 Jan 2020 07:12:50 -0800
From:   Eric Dumazet <>
To:     David Miller <>,
Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve

On 1/16/20 4:27 AM, David Miller wrote:
> From: Shaokun Zhang <>
> Date: Wed, 15 Jan 2020 11:23:40 +0800
>> From: Yuqi Jin <>
>> atomic_try_cmpxchg is called instead of atomic_cmpxchg that can reduce
>> the access number of the global variable @p_id in the loop. Let's
>> optimize it for performance.
>> Cc: "David S. Miller" <>
>> Cc: Alexey Kuznetsov <>
>> Cc: Hideaki YOSHIFUJI <>
>> Cc: Eric Dumazet <>
>> Cc: Yang Guo <>
>> Signed-off-by: Yuqi Jin <>
>> Signed-off-by: Shaokun Zhang <>
> I doubt this makes any measurable improvement in performance.
> If you can document a specific measurable improvement under
> a useful set of circumstances for real usage, then put those
> details into the commit message and resubmit.
> Otherwise, I'm not applying this, sorry.

Real difference that could be made here is to 
only use this cmpxchg() dance for CONFIG_UBSAN

When CONFIG_UBSAN is not set, atomic_add_return() is just fine.

(Supposedly UBSAN should not warn about that either, but this depends on compiler version)


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