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Message-ID: <bf33540b3f93b32b5680ec3cab0a005b996bef5f.camel@HansenPartnership.com>
Subject: Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable
 outside the loop
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Xiaomeng Tong <xiam0nd.tong@gmail.com>
Cc:     arnd@arndb.de, gregkh@linuxfoundation.org, jakobkoschel@gmail.com,
        jannh@google.com, keescook@chromium.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, netdev@vger.kernel.org,
        torvalds@linux-foundation.org
Date:   Sun, 06 Mar 2022 09:33:56 -0500
In-Reply-To: <20220303033122.10028-1-xiam0nd.tong@gmail.com>
References: <c0fc6e9c096778dce5c1e63c29af5ebdce83aca6.camel@HansenPartnership.com>
         <20220303033122.10028-1-xiam0nd.tong@gmail.com>
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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@HansenPartnership.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