linux-kernel - Re: [PATCH] tcp: avoid bogus warning in tcp_clean_rtx

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Date:   Tue, 19 Sep 2017 23:32:33 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Miller <davem@...emloft.net>
Cc:     Alexey Kuznetsov <kuznet@....inr.ac.ru>, yoshfuji@...ux-ipv6.org,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>, ycheng@...gle.com,
        soheil@...gle.com, Florian Westphal <fw@...len.de>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tcp: avoid bogus warning in tcp_clean_rtx_queue

On Tue, Sep 19, 2017 at 11:02 PM, David Miller <davem@...emloft.net> wrote:
> From: Arnd Bergmann <arnd@...db.de>
> Date: Mon, 18 Sep 2017 22:48:47 +0200
>
>> gcc-4.9 warns that it cannot trace the state of the 'last_ackt'
>> variable since the change to the TCP timestamping code, when
>> CONFIG_PROFILE_ANNOTATED_BRANCHES is set:
>>
>> net/ipv4/tcp_input.c: In function 'tcp_clean_rtx_queue':
>> include/net/tcp.h:757:23: error: 'last_ackt' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>>
>> Other gcc versions, both older and newer do now show this
>> warning. Removing the 'likely' annotation makes it go away,
>> and has no effect on the object code without
>> CONFIG_PROFILE_ANNOTATED_BRANCHES, as tested with gcc-4.9
>> and gcc-7.1.1, so this seems to be a safe workaround.
>>
>> Fixes: 9a568de4818d ("tcp: switch TCP TS option (RFC 7323) to 1ms clock")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> This reaches the limits at which I am willing to work around compiler
> stuff.

I see. It is a definitely a really obscure case, so if there is any doubt
that the workaround is harmless, then we shouldn't take it. The warning
only shows up on gcc-4.9 but not anything newer, and we disable
-Wmaybe-uninitialized on all older versions because of the false
positives.

It's also possible that it needed a combination of multiple other options,
not just CONFIG_PROFILE_ANNOTATED_BRANCHES. I build-tested
with gcc-4.9 to see if anything would show up that we don't also get a
warning for in gcc-7, and this came up once in several hundred randconfig
builds across multiple architectures (no other new warnings appeared
with gcc-4.9).

> What cpu did you test the object code generation upon and does that
> cpu have branch prediction hints in the target you are building for?

This was a randconfig build targetting ARMv5. I'm pretty sure that has
no such hint instructions.

       Arnd