From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: [PATCH-tip v7 09/15] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value Date: Wed, 18 Oct 2017 14:30:25 -0400 Message-ID: <1508351431-22375-10-git-send-email-longman@redhat.com> References: <1508351431-22375-1-git-send-email-longman@redhat.com> Return-path: In-Reply-To: <1508351431-22375-1-git-send-email-longman@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Davidlohr Bueso , Dave Chinner , Waiman Long 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 --- 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 15cdb28..7597736 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -298,9 +298,13 @@ 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); @@ -324,7 +328,7 @@ 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(); @@ -335,7 +339,7 @@ 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, @@ -356,7 +360,7 @@ 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