linux-kernel - Re: [PATCH v3 04/23] linux/net.h: use unique identifier for each struct

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Date:   Sat, 10 Nov 2018 00:31:47 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Joe Perches <joe@...ches.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 04/23] linux/net.h: use unique identifier for each
 struct _ddebug

On 10/11/2018 00.22, Joe Perches wrote:
> On Sat, 2018-11-10 at 00:10 +0100, Rasmus Villemoes wrote:
>> Changes on x86-64 later in this series require that all struct _ddebug
>> descriptors in a translation unit uses distinct identifiers. Realize
>> that for net_dbg_ratelimited by generating such an identifier via
>> __UNIQUE_ID and pass that to an extra level of macros.
>>
>> No functional change.
> 
> Why does this require an extra indirection _<macro>?
> 

Because the identifier used in the

>>  	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);

line must be the same as the one in the

>>  	if (DYNAMIC_DEBUG_BRANCH(descriptor) &&	

line. That identifier used to be simply "descriptor", with C scope rules
making that ok. But because of the asm games I'm playing later, I need
to generate a fresh identifier for each instance. That identifier must
be referenced in two places in the C source - hence the need to use an
extra level of macros to have the __UNIQUE_ID() expanded before
_net_dbg_ratelimited gets expanded.

More or less every use of __UNIQUE_ID requires such an extra level
(because one very rarely needs to use an identifier exactly once - the
point of giving stuff a name is to refer to it again.)

Rasmus