Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it is unlocked

Jason Low
2014-06-04T21:53:51Z

On Wed, 2014-06-04 at 13:57 -0700, Davidlohr Bueso wrote:
> On Wed, 2014-06-04 at 21:43 +0200, Peter Zijlstra wrote:
> > On Wed, Jun 04, 2014 at 12:08:29PM -0700, Jason Low wrote:
> > > Upon entering the slowpath in __mutex_lock_common(), we try once more
> > > to acquire the mutex. We only try to acquire it if MUTEX_SHOW_NO_WAITER
> > > (lock->count >= 0) is true in order to avoid using the atomic xchg()
> > > operation whenever it is not necessary. However, we really only need
> > > to try to acquire if the mutex is free (lock->count == 1).
> > >
> > > This patch changes it so that we only try-acquire the mutex upon
> > > entering the slowpath if it is unlocked, rather than if there are
> > > no waiters. This helps further reduce unncessary atomic xchg()
> > > operations. Furthermore, this patch introduces and uses a new
> > > MUTEX_IS_UNLOCKED() macro to improve readbability.
> > >
> > > Signed-off-by: Jason Low <jason.low2@hp.com>
> > > ---
> > > kernel/locking/mutex.c | 10 ++++++----
> > > 1 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > > index bc73d33..0925968 100644
> > > --- a/kernel/locking/mutex.c
> > > +++ b/kernel/locking/mutex.c
> > > @@ -48,9 +48,10 @@
> > >
> > > /*
> > > * A negative mutex count indicates that waiters are sleeping waiting for the
> > > - * mutex.
> > > + * mutex, and a count of one indicates the mutex is unlocked.
> > > */
> > > #define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0)
> > > +#define MUTEX_IS_UNLOCKED(mutex) (atomic_read(&(mutex)->count) == 1)
> >
> > So I recently saw that MUTEX_SHOW_NO_WAITER thing and cried a little;
> > and now you're adding more of that same nonsense.
> >
> > Please make them inline functions, also can we rename the SHOW_NO_WAITER
> > thing, because its not at all clear to me wtf it does; should it be
> > called: mutex_no_waiters() or somesuch?
>
> Agreed.
>
> In addition, how about the following helpers instead:
> - mutex_is_unlocked() : count > 0
> - mutex_has_waiters() : count < 0, or list_empty(->wait_list)

Sounds good. Likewise, for "mutex_is_unlocked()" I've noticed that there
is a mutex_is_locked() function provided in the linux/mutex.h file.
Perhaps we can just reuse that function and use !mutex_is_locked() for
places where we want to check if unlocked?