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From: Al Viro <viro@ZenIV.linux.org.uk>
Date: Tue, 17 Mar 2015 18:19:15 +0000
Subject: [Ocfs2-devel] [RFC PATCH 1/5] new helper: iov_iter_rw()
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To: dsterba-AlSwsSmVLrQ@public.gmane.org, Omar Sandoval <osandov-nWWhXC5lh1RBDgjK7y7TUQ@public.gmane.org>, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-btrfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, ceph-devel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-cifs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, osd-dev-yNzVSZO3znNg9hUCZPvPmw@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-f2fs-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, fuse-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, cluster-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, jfs-discussion-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, HPDD-discuss-y27Ovi1pjclAfugRpC6u6w@public.gmane.org, linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nilfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, ocfs2-devel-N0ozoZBvEnrZJqsBc5GL+g@public.gmane.org, reiserfs-devel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, v9fs-developer-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, xfs-VZNHf3L845pBDgjK7y7TUQ@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Chris Mason <clm-b10kYP2dOMg@public.gmane.org>, Josef Bacik <jbacik-b10kYP2dOMg@public.gmane.org>, Yan Zheng <zyan-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Sage Weil <sage-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Steve French <sfrench-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>, Boaz Harrosh <ooo-rh7Tgz9RNieUD9Wbbkgo/g@public.gmane.org>, Benny Halevy <bhalevy-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org>, Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>, Theodore Ts'o <tytso-3s7WtUTddSA@public.gmane.org>

On Tue, Mar 17, 2015 at 10:31:51AM +0100, David Sterba wrote:

> Agreed, but the proposed define is rather cryptic and I was not able to
> understand the meaning on the first glance.
> 
> > #define iov_iter_rw(i) ((0 ? (struct iov_iter *)0 : (i))->type & RW_MASK)
> 
> This worked for me, does not compile with anything else than
> 'struct iov_iter*' as i:
> 
> #define iov_iter_rw(i)	({			\
> 	struct iov_iter __iter = *(i);		\
> 	(i)->type & RW_MASK;			\
> 	})
> 
> The assignment is optimized out.

... and you are getting
	a) use of rather lousy gccism when plain C would do
	b) double evaluation since you've got it wrong (should've been
__iter.type & RW_MASK, if you do it that way).  As it is, if argument has
any side effects, your variant will trigger those twice - even if the
destination of the assignment is never used, the side effects remain.

I agree that it could use /* use ?: for typechecking */, but let's not go into
({...}) land unless we absolutely have to.