Date: Sun, 1 Nov 2009 15:15:44 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Ed Schouten <ed@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r198706 - head/sys/sys Message-ID: <alpine.BSF.2.00.0911011506130.2930@fledge.watson.org> In-Reply-To: <200910311035.n9VAZfIb082932@svn.freebsd.org> References: <200910311035.n9VAZfIb082932@svn.freebsd.org>
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On Sat, 31 Oct 2009, Ed Schouten wrote:
> Turn unused structure fields of cdevsw into spares.
>
> d_uid, d_gid and d_mode are unused, because permissions are stored in
> cdevpriv nowadays. d_kind doesn't seem to be used at all. We no longer
> keep a list of cdevsw's, so d_list is also unused.
>
> uid_t and gid_t are 32 bits, but mode_t is 16 bits, Because of alignment
> constraints of d_kind, we can safely turn it into three 32-bit integers.
> d_kind and d_list is equal in size to three pointers.
The underlying change seems fine, especially in -CURRENT, but I'm confused by
the paragraph here on alignment and safety. What do you mean by safe, and
does it matter? From a casual glance (perhaps mistaken), it looks like this
changes the KBI, so all modules declaring struct cdevsw will need to be
rebuilt.
Robert N M Watson
Computer Laboratory
University of Cambridge
>
> Discussed with: kib
>
> Modified:
> head/sys/sys/conf.h
>
> Modified: head/sys/sys/conf.h
> ==============================================================================
> --- head/sys/sys/conf.h Sat Oct 31 09:03:48 2009 (r198705)
> +++ head/sys/sys/conf.h Sat Oct 31 10:35:41 2009 (r198706)
> @@ -210,15 +210,13 @@ struct cdevsw {
> d_kqfilter_t *d_kqfilter;
> d_purge_t *d_purge;
> d_mmap_single_t *d_mmap_single;
> - uid_t d_uid;
> - gid_t d_gid;
> - mode_t d_mode;
> - const char *d_kind;
> +
> + int32_t d_spare0[3];
> + void *d_spare1[3];
>
> /* These fields should not be messed with by drivers */
> - LIST_ENTRY(cdevsw) d_list;
> LIST_HEAD(, cdev) d_devs;
> - int d_spare3;
> + int d_spare2;
> union {
> struct cdevsw *gianttrick;
> SLIST_ENTRY(cdevsw) postfree_list;
>
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