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SubjectRe: [PATCH] locking/rwsem: simplify the is-owner-spinnable checks
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On 05/18/2018 12:55 PM, Oleg Nesterov wrote:
> Add the trivial owner_on_cpu() helper for rwsem_can_spin_on_owner() and
> rwsem_spin_on_owner(), it also allows to make rwsem_can_spin_on_owner()
> a bit more clear.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> kernel/locking/rwsem-xadd.c | 25 +++++++++++++------------
> 1 file changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index a903367..3064c50 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -347,6 +347,15 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> }
> }
>
> +static inline bool owner_on_cpu(struct task_struct *owner)
> +{
> + /*
> + * As lock holder preemption issue, we both skip spinning if
> + * task is not on cpu or its cpu is preempted
> + */
> + return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
> +}
> +
> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> {
> struct task_struct *owner;
> @@ -359,17 +368,10 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>
> rcu_read_lock();
> owner = READ_ONCE(sem->owner);
> - if (!owner || !is_rwsem_owner_spinnable(owner)) {
> - ret = !owner; /* !owner is spinnable */
> - goto done;
> + if (owner) {
> + ret = is_rwsem_owner_spinnable(owner) &&
> + owner_on_cpu(owner);
> }
> -
> - /*
> - * As lock holder preemption issue, we both skip spinning if task is not
> - * on cpu or its cpu is preempted
> - */
> - ret = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
> -done:
> rcu_read_unlock();
> return ret;
> }
> @@ -398,8 +400,7 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
> * abort spinning when need_resched or owner is not running or
> * owner's cpu is preempted.
> */
> - if (!owner->on_cpu || need_resched() ||
> - vcpu_is_preempted(task_cpu(owner))) {
> + if (need_resched() || !owner_on_cpu(owner)) {
> rcu_read_unlock();
> return false;
> }

Acked-by: Waiman Long <longman@redhat.com>

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