linux-kernel - Re: [RFC PATCH 1/5] new helper: iov_iter

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Date:	Tue, 17 Mar 2015 03:18:35 -0700
From:	Omar Sandoval <osandov@...ndov.com>
To:	David Sterba <dsterba@...e.cz>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, ceph-devel@...r.kernel.org,
	linux-cifs@...r.kernel.org, osd-dev@...n-osd.org,
	linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
	fuse-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
	jfs-discussion@...ts.sourceforge.net, HPDD-discuss@...1.01.org,
	linux-nfs@...r.kernel.org, linux-nilfs@...r.kernel.org,
	ocfs2-devel@....oracle.com, reiserfs-devel@...r.kernel.org,
	v9fs-developer@...ts.sourceforge.net, xfs@....sgi.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] new helper: iov_iter_rw()

On Tue, Mar 17, 2015 at 10:31:51AM +0100, David Sterba wrote:
> On Mon, Mar 16, 2015 at 05:36:05PM +0000, Al Viro wrote:
> > On Mon, Mar 16, 2015 at 04:33:49AM -0700, Omar Sandoval wrote:
> > > Get either READ or WRITE out of iter->type.
> > 
> > Umm...  
> > 
> > > + * Get one of READ or WRITE out of iter->type without any other flags OR'd in
> > > + * with it.
> > > + */
> > > +static inline int iov_iter_rw(const struct iov_iter *i)
> > > +{
> > > +	return i->type & RW_MASK;
> > > +}
> > 
> > TBH, I would turn that into a macro.  Reason: indirect includes.
> 
> 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.

[-cc individual fs maintainers to avoid all of these email bounces,
should've looked a bit closer at that get_maintainer.pl output...]

I agree that this is a bit more readable, but it evaluates i twice.
That's an easy fix, just do __iter.type instead of (i)->type, but
there's still the possibility of someone passing in something called
__iter as i, and the fix for that tends to be "add more underscores". At
the very least, Al's macro could probably use a comment explaining
what's going on there, though.

-- 
Omar
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