On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote:
> The rwsem_can_spin_on_owner() function currently allows optimistic
> spinning only if the owner field is defined and is running. That is
> too conservative as it will cause some tasks to miss the opportunity
> of doing spinning in case the owner hasn't been able to set the owner
> field in time or the lock has just become available.
>
> This patch enables more aggressive use of optimistic spinning by
> assuming that the lock is spinnable unless proved otherwise.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
> kernel/locking/rwsem-xadd.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index d058946..dce22b8 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> {
> struct task_struct *owner;
> - bool on_cpu = false;
> + bool on_cpu = true; /* Assume spinnable unless proved not to be */
Nope, unfortunately we need as it fixes some pretty bad regressions when
dealing with multiple readers -- as readers do not deal with lock
ownership, so another thread can spin for too long in !owner. See commit
37e95624.
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