linux-kernel - Re: [PATCH 1/2] staging: rtl8723bs: Convert LIST_CONTAINOR to use kernel container

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Message-ID: <1497436701.18751.41.camel@perches.com>
Date:   Wed, 14 Jun 2017 03:38:21 -0700
From:   Joe Perches <joe@...ches.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        Hans de Goede <hdegoede@...hat.com>,
        Larry Finger <Larry.Finger@...inger.net>
Subject: Re: [PATCH 1/2] staging: rtl8723bs: Convert LIST_CONTAINOR to use
 kernel container_of

On Wed, 2017-06-14 at 12:18 +0200, Greg Kroah-Hartman wrote:
> On Tue, Jun 13, 2017 at 02:12:56PM -0700, Joe Perches wrote:
> > These are similar macros so use the normal kernel one.
> > 
> > As well, there are odd games being played with casting a plist to
> > a union recv_frame by using LIST_CONTAINOR.  Just use a direct cast
> > to union recv_frame instead.
[]
> > diff --git a/drivers/staging/rtl8723bs/core/rtw_recv.c b/drivers/staging/rtl8723bs/core/rtw_recv.c
[]
> > @@ -129,7 +129,7 @@ union recv_frame *_rtw_alloc_recvframe(struct __queue *pfree_recv_queue)
> >  
> >  		plist = get_next(phead);
> >  
> > -		precvframe = LIST_CONTAINOR(plist, union recv_frame, u);
> > +		precvframe = (union recv_frame *)plist;
> 
> No, you are "assuming" that the list_head is going to stay the first
> object of this structure, and what if it isn't?
> 
> Just use container_of, that way at least you get the type safeness of
> the call, and if something changes in the future, you don't instantly
> break the code everywhere without knowing it.

It's a named union u and it's exactly at the beginning.
It's unlikely to change.

The container_of macro doesn't work here as there it has
a BUILD_BUG_ON_MSG and there are pointer type mismatches
on those uses.