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Date:   Thu, 18 Aug 2022 10:09:46 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     menglong8.dong@gmail.com
Cc:     miguel.ojeda.sandonis@gmail.com, ojeda@kernel.org,
        ndesaulniers@google.com, davem@davemloft.net, edumazet@google.com,
        pabeni@redhat.com, asml.silence@gmail.com, imagedong@tencent.com,
        luiz.von.dentz@intel.com, vasily.averin@linux.dev,
        jk@codeconstruct.com.au, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, kernel test robot <lkp@intel.com>
Subject: Re: [PATCH net-next v4] net: skb: prevent the split of
 kfree_skb_reason() by gcc
Message-ID: <20220818100946.6ad96b06@kernel.org>
In-Reply-To: <20220816032846.2579217-1-imagedong@tencent.com>
References: <20220816032846.2579217-1-imagedong@tencent.com>
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On Tue, 16 Aug 2022 11:28:46 +0800 menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.com>
> 
> Sometimes, gcc will optimize the function by spliting it to two or
> more functions. In this case, kfree_skb_reason() is splited to
> kfree_skb_reason and kfree_skb_reason.part.0. However, the
> function/tracepoint trace_kfree_skb() in it needs the return address
> of kfree_skb_reason().
> 
> This split makes the call chains becomes:
>   kfree_skb_reason() -> kfree_skb_reason.part.0 -> trace_kfree_skb()
> 
> which makes the return address that passed to trace_kfree_skb() be
> kfree_skb().
> 
> Therefore, prevent this kind of optimization to kfree_skb_reason() by
> making the optimize level to "O1". I think these should be better
> method instead of this "O1", but I can't figure it out......
> 
> This optimization CAN happen, which depend on the behavior of gcc.
> I'm not able to reproduce it in the latest kernel code, but it happens
> in my kernel of version 5.4.119. Maybe the latest code already do someting
> that prevent this happen?
> 
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

Sorry for a late and possibly off-topic chime in, is the compiler
splitting it because it thinks that skb_unref() is going to return 
true? I don't think that's the likely case, so maybe we're better 
off wrapping that skb_unref() in unlikely()?