Re: [PATCH 1/2] locking/rwsem: Add CONFIG_RWSEM_SPIN_ON_OWNER

Jason Low
2014-06-06T19:09:10Z

On Fri, 2014-06-06 at 11:48 -0700, Davidlohr Bueso wrote:
> On Fri, 2014-06-06 at 11:12 -0700, Davidlohr Bueso wrote:
> > On Fri, 2014-06-06 at 10:13 -0700, Jason Low wrote:
> > > On Fri, 2014-06-06 at 08:55 -0700, Davidlohr Bueso wrote:
> > > > Just like with mutexes (CONFIG_MUTEX_SPIN_ON_OWNER),
> > > > encapsulate the dependencies for rwsem optimistic spinning.
> > > > No logical changes here as it continues to depend on both
> > > > SMP and the XADD algorithm variant.
> > > >
> > > > Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> > > > ---
> > > > include/linux/rwsem.h | 4 ++--
> > > > kernel/Kconfig.locks | 4 ++++
> > > > kernel/locking/rwsem.c | 2 +-
> > > > 3 files changed, 7 insertions(+), 3 deletions(-)
> > >
> > > Do we also want to add an #ifdef CONFIG_RWSEM_SPIN_ON_OWNER in
> > > __init_rwsem() and in the optimistic spinning functions for rwsem-xadd?
> > >
> >
> > Not really, as we conditionally build rwsem-xadd.o based on
> > CONFIG_RWSEM_XCHGADD_ALGORITHM:
> >
> > obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
> > obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
> >
> > So leaving just SMP is ok there.
>
> Of course that's bogus when we add more dependencies (ie 2/2), so yeah,
> we need to add the whole CONFIG_RWSEM_SPIN_ON_OWNER option.
>
> Thanks.

> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index dacc321..f6b5b96 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -82,7 +82,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
> sem->count = RWSEM_UNLOCKED_VALUE;
> raw_spin_lock_init(&sem->wait_lock);
> INIT_LIST_HEAD(&sem->wait_list);
> -#ifdef CONFIG_SMP
> +#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> sem->owner = NULL;
> sem->osq = NULL;
> #endif

And should we also change that in the optimistic spinning functions so
that it defaults to:

static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
return false;
}

in the !CONFIG_RWSEM_SPIN_ON_OWNER case.

Thanks.

> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 42f806d..e2d3bc7 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -12,7 +12,7 @@
>
> #include <linux/atomic.h>
>
> -#if defined(CONFIG_SMP) && defined(CONFIG_RWSEM_XCHGADD_ALGORITHM)
> +#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> static inline void rwsem_set_owner(struct rw_semaphore *sem)
> {
> sem->owner = current;