netdev - Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb

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Message-ID: <87zgfbnh81.fsf@oldenburg.str.redhat.com>
Date:   Wed, 07 Sep 2022 20:59:26 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Menglong Dong <menglong8.dong@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>, kuba@...nel.org,
        miguel.ojeda.sandonis@...il.com, ojeda@...nel.org,
        davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        asml.silence@...il.com, imagedong@...cent.com,
        luiz.von.dentz@...el.com, vasily.averin@...ux.dev,
        jk@...econstruct.com.au, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel test robot <lkp@...el.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH net-next v4] net: skb: prevent the split of
 kfree_skb_reason() by gcc

* Segher Boessenkool:

> On Tue, Sep 06, 2022 at 02:37:47PM +0200, Florian Weimer wrote:
>> > On Mon, Aug 22, 2022 at 4:01 PM Florian Weimer <fweimer@...hat.com> wrote:
>> > I did some research on the 'sibcalls' you mentioned above. Feel like
>> > It's a little similar to 'inline', and makes the callee use the same stack
>> > frame with the caller, which obviously will influence the result of
>> > '__builtin_return_address'.
>
> Sibling calls are essentially calls that can be replaced by jumps (aka
> "tail call"), without needing a separate entry point to the callee.
>
> Different targets can have a slightly different implementation and
> definition of what exactly is a sibling call, but that's the gist.
>
>> > Hmm......but I'm not able to find any attribute to disable this optimization.
>> > Do you have any ideas?
>> 
>> Unless something changed quite recently, GCC does not allow disabling
>> the optimization with a simple attribute (which would have to apply to
>> function pointers as well, not functions).
>
> It isn't specified what a sibling call exactly *is*, certainly not on C
> level (only in the generated machine code), and the details differs per
> target.

Sure, but GCC already disables this optimization in a generic fashion
for noreturn calls.  It should be possible to do the same based another
function attribute.

Thanks,
Florian