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

Davidlohr Bueso
2014-06-04T20:57:09Z

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)

If combined with the negation, I think that pretty much covers the whole
life-cycle of a mutex. The mutex.c file is full of places where we can
replace crude atomic_reads with these.

Thanks,
Davidlohr