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Date:   Wed, 20 May 2020 10:39:33 +0200
From:   Antonio Quartulli <>
To:     David Miller <>
Subject: Re: [PATCH] net/sch_generic.h: use sizeof_member() and get rid of
 unused variable

Hi David,

On 20/05/2020 00:40, David Miller wrote:
> From: Antonio Quartulli <>
> Date: Tue, 19 May 2020 11:13:33 +0200
>> Compiling with -Wunused triggers the following warning:
>> ./include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
>> ./include/net/sch_generic.h:464:23: warning: unused variable ‘qcb’ [-Wunused-variable]
>>   464 |  struct qdisc_skb_cb *qcb;
>>       |                       ^~~
>> as the qcb variable is only used to compute the sizeof one of its members.
> It's referenced in the code, therefore it is not "unused".


> If in some configuration BUILD_BUG_ON() does not reference it's arguments,
> that's the bug that needs to be fixed.

I don't think it's BUILD_BUG_ON()'s fault, because qcb->data is passed
to sizeof() first.

My best guess is that gcc is somewhat optimizing the sizeof(gcb->data)
and thus leaving the gcb variable unused.

This said, I think it's better for the code style (and for the compiler)
if we used sizeof_member().

Should I resend the patch with a commit message that does not mention
the "unused" warning?

Thanks a lot.
Best Regards,

Antonio Quartulli

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