Re: [PATCH 2/4] locking/rtmutex: Use cmp-cmpxchg

Davidlohr Bueso
2015-06-06T15:28:02Z

On Fri, 2015-06-05 at 14:38 +0200, Thomas Gleixner wrote:
> On Tue, 19 May 2015, Davidlohr Bueso wrote:
>
> > Avoid unnecessary cmpxchg calls, all of our other locks
> > use it as well.
> >
> > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> > ---
> > kernel/locking/rtmutex.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > index 74188d8..1d5cc9d 100644
> > --- a/kernel/locking/rtmutex.c
> > +++ b/kernel/locking/rtmutex.c
> > @@ -74,7 +74,9 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
> > * set up.
> > */
> > #ifndef CONFIG_DEBUG_RT_MUTEXES
> > -# define rt_mutex_cmpxchg(l,c,n) (cmpxchg(&l->owner, c, n) == c)
> > +# define rt_mutex_cmpxchg(l,c,n) \
> > + (l->owner == c && cmpxchg(&l->owner, c, n) == c)
>
> Errm. How does that improve stuff like rt_mutex_lock() ?

It just avoids a cmpxchg in the fastpath when locked, at the cost of an
extra test when unlocked. CCAS techniques have been proven to boost some
workloads for both rwsems and mutexes. That's all.

Thanks,
Davidlohr