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    Subject[PATCH-tip v7 09/15] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value
    Date
    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@redhat.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 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
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