Commit-ID: ddd0fa73c2b71c35de4fe7ae60a5f1a6cddc2cf0 Gitweb: http://git.kernel.org/tip/ddd0fa73c2b71c35de4fe7ae60a5f1a6cddc2cf0 Author: Waiman Long <waiman.l...@hpe.com> AuthorDate: Tue, 17 May 2016 21:26:23 -0400 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Wed, 8 Jun 2016 15:17:00 +0200
locking/rwsem: Streamline the rwsem_optimistic_spin() code This patch moves the owner loading and checking code entirely inside of rwsem_spin_on_owner() to simplify the logic of rwsem_optimistic_spin() loop. Suggested-by: Peter Hurley <pe...@hurleysoftware.com> Signed-off-by: Waiman Long <waiman.l...@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Reviewed-by: Peter Hurley <pe...@hurleysoftware.com> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Dave Chinner <da...@fromorbit.com> Cc: Davidlohr Bueso <d...@stgolabs.net> Cc: Douglas Hatch <doug.ha...@hpe.com> Cc: Jason Low <jason.l...@hp.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Scott J Norton <scott.nor...@hpe.com> Cc: Thomas Gleixner <t...@linutronix.de> Link: http://lkml.kernel.org/r/1463534783-38814-6-git-send-email-waiman.l...@hpe.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/locking/rwsem-xadd.c | 38 ++++++++++++++++++++------------------ 1 file changed, 20 insertions(+), 18 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 4f1daf5..2031281 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -350,9 +350,16 @@ done: return ret; } -static noinline -bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) +/* + * Return true only if we can still spin on the owner field of the rwsem. + */ +static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) { + struct task_struct *owner = READ_ONCE(sem->owner); + + if (!rwsem_owner_is_writer(owner)) + goto out; + rcu_read_lock(); while (sem->owner == owner) { /* @@ -372,7 +379,7 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) cpu_relax_lowlatency(); } rcu_read_unlock(); - +out: /* * If there is a new owner or the owner is not set, we continue * spinning. @@ -382,7 +389,6 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) static bool rwsem_optimistic_spin(struct rw_semaphore *sem) { - struct task_struct *owner; bool taken = false; preempt_disable(); @@ -394,21 +400,17 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) if (!osq_lock(&sem->osq)) goto done; - while (true) { - owner = READ_ONCE(sem->owner); + /* + * Optimistically spin on the owner field and attempt to acquire the + * lock whenever the owner changes. Spinning will be stopped when: + * 1) the owning writer isn't running; or + * 2) readers own the lock as we can't determine if they are + * actively running or not. + */ + while (rwsem_spin_on_owner(sem)) { /* - * Don't spin if - * 1) the owner is a reader as we we can't determine if the - * reader is actively running or not. - * 2) The rwsem_spin_on_owner() returns false which means - * the owner isn't running. + * Try to acquire the lock */ - if (rwsem_owner_is_reader(owner) || - (rwsem_owner_is_writer(owner) && - !rwsem_spin_on_owner(sem, owner))) - break; - - /* wait_lock will be acquired if write_lock is obtained */ if (rwsem_try_write_lock_unqueued(sem)) { taken = true; break; @@ -420,7 +422,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) * we're an RT task that will live-lock because we won't let * the owner complete. */ - if (!owner && (need_resched() || rt_task(current))) + if (!sem->owner && (need_resched() || rt_task(current))) break; /*
