From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753260AbbCQSVu (ORCPT ); Tue, 17 Mar 2015 14:21:50 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:39303 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752044AbbCQSVo (ORCPT ); Tue, 17 Mar 2015 14:21:44 -0400 Date: Tue, 17 Mar 2015 18:19:15 +0000 From: Al Viro To: dsterba@suse.cz, Omar Sandoval , linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, linux-cifs@vger.kernel.org, osd-dev@open-osd.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, fuse-devel@lists.sourceforge.net, cluster-devel@redhat.com, jfs-discussion@lists.sourceforge.net, HPDD-discuss@ml01.01.org, linux-nfs@vger.kernel.org, linux-nilfs@vger.kernel.org, ocfs2-devel@oss.oracle.com, reiserfs-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, xfs@oss.sgi.com, linux-kernel@vger.kernel.org, Chris Mason , Josef Bacik , Yan Zheng , Sage Weil , Steve French , Boaz Harrosh , Benny Halevy , Jan Kara , "Theodore Ts'o" , Andreas Dilger , Jaegeuk Kim , Changman Lee , Miklos Szeredi , Steven Whitehouse , Dave Kleikamp , Oleg Drokin , Trond Myklebust , Anna Schumaker , Ryusuke Konishi , Mark Fasheh , Joel Becker , Eric Van Hensbergen , Ron Minnich , Latchesar Ionkov , Dave Chinner Subject: Re: [RFC PATCH 1/5] new helper: iov_iter_rw() 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 Content-Disposition: inline In-Reply-To: <20150317093151.GS20767@twin.jikos.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@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.