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

Peter Zijlstra
2014-06-04T19:43:41Z

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?