linux-kernel - Re: [PATCH 2/2] Introduce __cond_lock

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Date:   Thu, 21 Dec 2017 14:00:16 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 2/2] Introduce __cond_lock_err

On Thu, Dec 21, 2017 at 02:48:10PM -0700, Ross Zwisler wrote:
> On Tue, Dec 19, 2017 at 08:58:23AM -0800, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@...rosoft.com>
> > 
> > The __cond_lock macro expects the function to return 'true' if the lock
> > was acquired and 'false' if it wasn't.  We have another common calling
> > convention in the kernel, which is returning 0 on success and an errno
> > on failure.  It's hard to use the existing __cond_lock macro for those
> > kinds of functions, so introduce __cond_lock_err() and convert the
> > two existing users.
> 
> This is much cleaner!  One quick issue below.
> 
> > Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
> > ---
> >  include/linux/compiler_types.h | 2 ++
> >  include/linux/mm.h             | 9 ++-------
> >  mm/memory.c                    | 9 ++-------
> >  3 files changed, 6 insertions(+), 14 deletions(-)
> > 
> > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > index 6b79a9bba9a7..ff3c41c78efa 100644
> > --- a/include/linux/compiler_types.h
> > +++ b/include/linux/compiler_types.h
> > @@ -16,6 +16,7 @@
> >  # define __acquire(x)	__context__(x,1)
> >  # define __release(x)	__context__(x,-1)
> >  # define __cond_lock(x,c)	((c) ? ({ __acquire(x); 1; }) : 0)
> > +# define __cond_lock_err(x,c)	((c) ? 1 : ({ __acquire(x); 0; }))
> 					       ^
> I think we actually want this to return c here ^

Then you want to use ((c) ?: ...), to avoid evaluating c twice.

- Josh Triplett