From: Al Viro Subject: Re: [RFC PATCH 1/5] new helper: iov_iter_rw() Date: Tue, 17 Mar 2015 18:19:15 +0000 Message-ID: <20150317181910.GK29656@ZenIV.linux.org.uk> References: <34dc78b262546e9343e0ed872232a97f5eaa5f15.1426502566.git.osandov@osandov.com> <20150316173605.GX29656@ZenIV.linux.org.uk> <20150317093151.GS20767@twin.jikos.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: dsterba-AlSwsSmVLrQ@public.gmane.org, Omar Sandoval , 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 , Josef Bacik , Yan Zheng , Sage Weil , Steve French , Boaz Harrosh , Benny Halevy , Jan Kara , Theodore Ts'o , Andreas Dilger < Return-path: Content-Disposition: inline In-Reply-To: <20150317093151.GS20767-1ReQVI26iDCaZKY3DrU6dA@public.gmane.org> Sender: linux-nilfs-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-ext4.vger.kernel.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. -- To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html