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Date: Thu, 1 Jun 2017 13:39:05 -0400 From: Waiman Long <longman@...hat.com> To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com> Cc: linux-kernel@...r.kernel.org, x86@...nel.org, linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org, linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>, Dave Chinner <david@...morbit.com>, Waiman Long <longman@...hat.com> Subject: [PATCH v5 7/9] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value This patch modifies rwsem_spin_on_owner() to return a tri-state value to better reflect the state of lock holder which enables us to make a better decision of what to do next. Signed-off-by: Waiman Long <longman@...hat.com> --- kernel/locking/rwsem-xadd.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index f82ce29..7d030a2 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -341,9 +341,13 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) } /* - * Return true only if we can still spin on the owner field of the rwsem. + * Return the folowing three values depending on the lock owner state. + * 1 when owner has changed and no reader is detected yet. + * 0 when owner has change and/or owner is a reader. + * -1 when optimistic spinning has to stop because either the owner stops + * running or its timeslice has been used up. */ -static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) +static noinline int rwsem_spin_on_owner(struct rw_semaphore *sem) { struct task_struct *owner = READ_ONCE(sem->owner); @@ -367,7 +371,7 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) if (!owner->on_cpu || need_resched() || vcpu_is_preempted(task_cpu(owner))) { rcu_read_unlock(); - return false; + return -1; } cpu_relax(); @@ -378,7 +382,7 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) * If there is a new owner or the owner is not set, we continue * spinning. */ - return !rwsem_owner_is_reader(READ_ONCE(sem->owner)); + return rwsem_owner_is_reader(READ_ONCE(sem->owner)) ? 0 : 1; } static bool rwsem_optimistic_spin(struct rw_semaphore *sem, @@ -399,7 +403,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, * 2) readers own the lock as we can't determine if they are * actively running or not. */ - while (rwsem_spin_on_owner(sem)) { + while (rwsem_spin_on_owner(sem) > 0) { /* * Try to acquire the lock */ -- 1.8.3.1
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