linux-kernel - Re: [PATCH] linux/kernel.h: add container

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Message-ID: <20200827213636.GF1236603@ZenIV.linux.org.uk>
Date:   Thu, 27 Aug 2020 22:36:36 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Allen Pais <allen.lkml@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] linux/kernel.h: add container_from()

On Thu, Aug 27, 2020 at 01:46:33PM -0700, Linus Torvalds wrote:

> You really have to pick some pretty excessive type names (or variable
> names) to get close to 80 characters. Again, to pick an example:
> 
>         struct timer_group_priv *priv = container_of(handle,
>                         struct timer_group_priv, timer[handle->num]);
> 
> ends up being long even if you were to split it, but that funky
> container_from() wouldn't have helped the real problem - the fact that
> the above is complex and nasty.
> 
> And I had to _search_ for that example. All the normal cases of
> split-line container-of's were due to doing it with the declaration,
> or beause the first argument ended up being an expression in itself
> and the nested expressions made it more complex.

Speaking of searching, this kind of typeof use is, IMO, actively
harmful - it makes finding the places where we might get from
e.g. linked list to containing objects much harder.  container_of
(unless combined with obfuscating use of typeof()) at least gives
you a chance to grep - struct foo *not* followed by '*' is a pattern
that doesn't give too many false positives.  This one, OTOH, is
essentially impossible to grep for.