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

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Date:   Thu, 21 Dec 2017 15:10:50 -0700
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        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 02:00:16PM -0800, Josh Triplett wrote:
> 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.

Oh, yep, great catch.