linux-kernel - Re: [PATCH net-next 2/2] netdev_queues: fix -Wshadow / Sparse shadow warnings throughout the file

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Date: Tue, 2 Apr 2024 13:53:54 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Dmitry Safonov
	<0x7f454c46@...il.com>, Heiner Kallweit <hkallweit1@...il.com>,
	<nex.sw.ncis.osdt.itp.upstreaming@...el.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/2] netdev_queues: fix -Wshadow / Sparse shadow
 warnings throughout the file

From: Jakub Kicinski <kuba@...nel.org>
Date: Fri, 29 Mar 2024 13:53:44 -0700

> On Fri, 29 Mar 2024 13:18:57 -0700 Jakub Kicinski wrote:
>>> Sparse:
>>>
>>> drivers/net/ethernet/intel/idpf/idpf_txrx.c:1992:16: warning: symbol '_res' shadows an earlier one
>>> drivers/net/ethernet/intel/idpf/idpf_txrx.c:1992:16: originally declared here  
>>
>> I don't see these building with LLVM=1 W=12 C=1
>> and I really don't like complicating the code because the compiler
>> is stupid. Can't you solve this with some renames? Add another

It's not the compiler, its warnings are valid actually. Shadowing makes
it very easy to confuse variables and make bugs...

>> underscore or something?
> 
> I'm stupid I tried on the test branch which already had your fix..

:D Sometimes it happens.

> 
> This is enough:
> 
> diff --git a/include/net/netdev_queues.h b/include/net/netdev_queues.h
> index 1ec408585373..2270fbb99cf7 100644
> --- a/include/net/netdev_queues.h
> +++ b/include/net/netdev_queues.h
> @@ -89,7 +89,7 @@ struct netdev_stat_ops {
>  
>  #define netif_txq_try_stop(txq, get_desc, start_thrs)			\
>  	({								\
> -		int _res;						\
> +		int __res;						\
>  									\
>  		netif_tx_stop_queue(txq);				\
>  		/* Producer index and stop bit must be visible		\
> @@ -101,12 +101,12 @@ struct netdev_stat_ops {
>  		/* We need to check again in a case another		\
>  		 * CPU has just made room available.			\
>  		 */							\
> -		_res = 0;						\
> +		__res = 0;						\
>  		if (unlikely(get_desc >= start_thrs)) {			\
>  			netif_tx_start_queue(txq);			\
> -			_res = -1;					\
> +			__res = -1;					\
>  		}							\
> -		_res;							\
> +		__res;							\
>  	})								\
>  
>  /**

But what if there's a function which calls one of these functions and
already has _res or __res or something? I know renaming is enough for
the warnings I mentioned, but without __UNIQUE_ID() anything can happen
anytime, so I wanted to fix that once and for all :z

I already saw some macros which have a layer of indirection for
__UNIQUE_ID(), but previously they didn't and then there were fixes
which added underscores, renamed variables etc etc...

Thanks,
Olek