linux-kernel - Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable outside the loop

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Date:   Sun, 06 Mar 2022 09:33:56 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     arnd@...db.de, gregkh@...uxfoundation.org, jakobkoschel@...il.com,
        jannh@...gle.com, keescook@...omium.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable
 outside the loop

On Thu, 2022-03-03 at 11:31 +0800, Xiaomeng Tong wrote:
> On Wed, 02 Mar 2022 08:02:23 -0500, James Bottomley
> <James.Bottomley@...senPartnership.com> wrote:
> > pos shouldn't be an input to the macro since it's being declared
> > inside
> > it.  All that will do will set up confusion about the shadowing of
> > pos.
> > The macro should still work as
> > 
> > #define list_for_each_entry_inside(type, head, member) \
> >   ...
> > 
> > For safety, you could
> > 
> > #define POS __UNIQUE_ID(pos)
> > 
> > and use POS as the loop variable .. you'll have to go through an
> > intermediate macro to get it to be stable.  There are examples in
> > linux/rcupdate.h
> 
> The outer "pos" variable is no longer needed and thus the declare
> statement before the loop is removed, see the demostration in PATCH
> 3~6. Now, there is only one inner "pos" variable left. Thus, there
> should be no such *shadow* problem.

So why is pos in the signature of your #define then?  Because that
means it expands to whatever goes in the first field of
list_for_each_entry_inside().

If someone needs to specify a unique name to avoid shadowing an
existing variable, then hide pos and use UNIQUE_ID instead was the
whole thrust of this comment.

James