linux-kernel - Re: [PATCH] gfs2: Use container_of() for gfs2

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Date:   Tue, 10 May 2022 13:13:31 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Kees Cook <keescook@...omium.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Bob Peterson <rpeterso@...hat.com>,
        kernel test robot <lkp@...el.com>,
        Bill Wendling <morbo@...gle.com>,
        cluster-devel <cluster-devel@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gfs2: Use container_of() for gfs2_glock(aspace)

Kees and Christoph,

thanks for the patch and review.

On Tue, May 10, 2022 at 9:29 AM Christoph Hellwig <hch@...radead.org> wrote:
> > +/* gfs2_glock_get(), "glock" must be first. */
> > +struct glock_aspace {
> > +     struct gfs2_glock       glock;
> > +     struct address_space    mapping;
> > +};
>
> Why does glock need to be first?  The whole point of using container_of
> is that we don't have to make that assumption.

Just needs to be cleaned up in gfs2_glock_get() and
gfs2_glock_dealloc() as well. I'll do that when applying the patch.

> >  static inline struct address_space *gfs2_glock2aspace(struct gfs2_glock *gl)
> >  {
> >       if (gl->gl_ops->go_flags & GLOF_ASPACE)
> > -             return (struct address_space *)(gl + 1);
> > +             return &(container_of(gl, struct glock_aspace, glock)->mapping);
>
> Do we even need the braces here?

Will use a local variable here, as suggested below.

> >       struct inode *inode = mapping->host;
> >       if (mapping->a_ops == &gfs2_meta_aops)
> > -             return (((struct gfs2_glock *)mapping) - 1)->gl_name.ln_sbd;
> > +             return container_of(mapping, struct glock_aspace, mapping)->glock.gl_name.ln_sbd;
>
> A local variable would be really nice for the reader here to decompose
> this a bit:
>
>                 struct glock_aspace *a =
>                         container_of(mapping, struct glock_aspace, mapping);
>
>                 return a->glock.gl_name.ln_sbd;

Yes.

Thanks,
Andreas