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

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Message-ID: <20150317181910.GK29656@ZenIV.linux.org.uk>
Date:	Tue, 17 Mar 2015 18:19:15 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
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Subject: Re: [RFC PATCH 1/5] new helper: iov_iter_rw()

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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