linux-kernel - Re: [PATCH -V2 01/13] mutex: add atomic_dec_and_mutex

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Date:	Tue, 28 Apr 2009 13:08:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	eparis@...hat.com, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, hch@...radead.org,
	alan@...rguk.ukuu.org.uk, sfr@...b.auug.org.au,
	john@...nmccutchan.com, rlove@...ve.org
Subject: Re: [PATCH -V2 01/13] mutex: add atomic_dec_and_mutex_lock

On Tue, 7 Apr 2009 16:06:01 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 27 Mar 2009 16:05:08 -0400
> Eric Paris <eparis@...hat.com> wrote:
> 
> > Much like the atomic_dec_and_lock() function in which we take an hold a
> > spin_lock if we drop the atomic to 0 this function takes and holds the
> > mutex if we dec the atomic to 0.
> > 
> > Signed-off-by: Eric Paris <eparis@...hat.com>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > LKML-Reference: <20090323172417.410913479@...llo.nl>
> > Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > ---
> > 
> >  include/linux/mutex.h |   23 +++++++++++++++++++++++
> >  1 files changed, 23 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> > index 3069ec7..93054fc 100644
> > --- a/include/linux/mutex.h
> > +++ b/include/linux/mutex.h
> > @@ -151,4 +151,27 @@ extern int __must_check mutex_lock_killable(struct mutex *lock);
> >  extern int mutex_trylock(struct mutex *lock);
> >  extern void mutex_unlock(struct mutex *lock);
> >  
> > +/**
> > + * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
> > + * @cnt: the atomic which we are to dec
> > + * @lock: the mutex to return holding if we dec to 0
> > + *
> > + * return true and hold lock if we dec to 0, return false otherwise
> > + */
> > +static inline int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
> > +{
> > +	/* dec if we can't possibly hit 0 */
> > +	if (atomic_add_unless(cnt, -1, 1))
> > +		return 0;
> > +	/* we might hit 0, so take the lock */
> > +	mutex_lock(lock);
> > +	if (!atomic_dec_and_test(cnt)) {
> > +		/* when we actually did the dec, we didn't hit 0 */
> > +		mutex_unlock(lock);
> > +		return 0;
> > +	}
> > +	/* we hit 0, and we hold the lock */
> > +	return 1;
> > +}
> > +
> 
> This looks too large to be inlined?

It still looks too large to be inlined.

Take a look at atomic_add_unless(), and split your sides laughing.

Once you factor in all the lockdep and other debug goop, the code
generation here is pretty bewildering, but it won't be small.

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