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DateTue, 28 Apr 2009 13:08:47 -0700
FromAndrew Morton <>
SubjectRe: [PATCH -V2 01/13] mutex: add atomic_dec_and_mutex_lock
On Tue, 7 Apr 2009 16:06:01 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 27 Mar 2009 16:05:08 -0400
> Eric Paris <eparis@redhat.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@redhat.com>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Paul Mackerras <paulus@samba.org>
> > LKML-Reference: <20090323172417.410913479@chello.nl>
> > Signed-off-by: Ingo Molnar <mingo@elte.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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