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.l...@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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
