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Re: [RFC PATCH 1/5] new helper: iov_iter_rw()

To: dsterba@xxxxxxx, Omar Sandoval <osandov@xxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, ceph-devel@xxxxxxxxxxxxxxx, linux-cifs@xxxxxxxxxxxxxxx, osd-dev@xxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx, fuse-devel@xxxxxxxxxxxxxxxxxxxxx, cluster-devel@xxxxxxxxxx, jfs-discussion@xxxxxxxxxxxxxxxxxxxxx, HPDD-discuss@xxxxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-nilfs@xxxxxxxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx, reiserfs-devel@xxxxxxxxxxxxxxx, v9fs-developer@xxxxxxxxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Chris Mason <clm@xxxxxx>, Josef Bacik <jbacik@xxxxxx>, Yan Zheng <zyan@xxxxxxxxxx>, Sage Weil <sage@xxxxxxxxxx>, Steve French <sfrench@xxxxxxxxx>, Boaz Harrosh <ooo@xxxxxxxxxxxxxxx>, Benny Halevy <bhalevy@xxxxxxxxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Theodore Ts'o <tytso@xxxxxxx>, Andreas Dilger <adilger.kernel@xxxxxxxxx>, Jaegeuk Kim <jaegeuk@xxxxxxxxxx>, Changman Lee <cm224.lee@xxxxxxxxxxx>, Miklos Szeredi <miklos@xxxxxxxxxx>, Steven Whitehouse <swhiteho@xxxxxxxxxx>, Dave Kleikamp <shaggy@xxxxxxxxxx>, Oleg Drokin <oleg.drokin@xxxxxxxxx>, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>, Anna Schumaker <anna.schumaker@xxxxxxxxxx>, Ryusuke Konishi <konishi.ryusuke@xxxxxxxxxxxxx>, Mark Fasheh <mfasheh@xxxxxxxx>, Joel Becker <jlbec@xxxxxxxxxxxx>, Eric Van Hensbergen <ericvh@xxxxxxxxx>, Ron Minnich <rminnich@xxxxxxxxxx>, Latchesar Ionkov <lucho@xxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC PATCH 1/5] new helper: iov_iter_rw()
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Tue, 17 Mar 2015 18:19:15 +0000
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150317093151.GS20767@xxxxxxxxxxxxx>
References: <cover.1426502566.git.osandov@xxxxxxxxxxx> <34dc78b262546e9343e0ed872232a97f5eaa5f15.1426502566.git.osandov@xxxxxxxxxxx> <20150316173605.GX29656@xxxxxxxxxxxxxxxxxx> <20150317093151.GS20767@xxxxxxxxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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.

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