Messages in this thread |  | | Date | Wed, 11 Jun 2014 21:28:33 -0400 | From | "Long, Wai Man" <> | Subject | Re: [PATCH v2 3/4] mutex: Try to acquire mutex only if it is unlocked |
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On 6/11/2014 2:37 PM, Jason Low wrote: > Upon entering the slowpath in __mutex_lock_common(), we try once more to > acquire the mutex. We only try to acquire if (lock->count >= 0). However, > what we actually want here is to try to acquire if the mutex is unlocked > (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 the lock count is non-negative. > This helps further reduce unnecessary atomic xchg() operations. > > Furthermore, this patch uses !mutex_is_locked(lock) to do the initial > checks for if the lock is free rather than directly calling atomic_read() > on the lock->count, in order to improve readability. > > Signed-off-by: Jason Low <jason.low2@hp.com> > --- > kernel/locking/mutex.c | 7 ++++--- > 1 files changed, 4 insertions(+), 3 deletions(-) > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > index 4bd9546..e4d997b 100644 > --- a/kernel/locking/mutex.c > +++ b/kernel/locking/mutex.c > @@ -432,7 +432,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > if (owner && !mutex_spin_on_owner(lock, owner)) > break; > > - if ((atomic_read(&lock->count) == 1) && > + /* Try to acquire the mutex if it is unlocked. */ > + if (!mutex_is_locked(lock) && > (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { > lock_acquired(&lock->dep_map, ip); > if (use_ww_ctx) { > @@ -479,9 +480,9 @@ slowpath: > > /* > * Once more, try to acquire the lock. Only try-lock the mutex if > - * lock->count >= 0 to reduce unnecessary xchg operations. > + * it is unlocked to reduce unnecessary xchg() operations. > */ > - if (atomic_read(&lock->count) >= 0 && (atomic_xchg(&lock->count, 0) == 1)) > + if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1)) > goto skip_wait; > > debug_mutex_lock_common(lock, &waiter);
Acked-by: Waiman Long <Waiman.Long@hp.com>
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