On Fri, May 06, 2016 at 08:20:24PM -0400, Waiman Long wrote:
> @@ -391,9 +386,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore
> *sem)
> * When there's no owner, we might have preempted between the
> * owner acquiring the lock and setting the owner field. If
> * we're an RT task that will live-lock because we won't let
> + * the owner complete. We also quit if the lock is owned by
> + * readers.
Maybe also note why we quit on readers.
> */
> + if (rwsem_is_reader_owned(owner) ||
> + (!owner && (need_resched() || rt_task(current))))
> break;
>
> /*
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index 870ed9a..d7fea18 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -1,3 +1,20 @@
> +/*
> + * The owner field of the rw_semaphore structure will be set to
> + * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
> + * the owner field when it unlocks. A reader, on the other hand, will
> + * not touch the owner field when it unlocks.
> + *
> + * In essence, the owner field now has the following 3 states:
> + * 1) 0
> + * - lock is free or the owner hasn't set the field yet
> + * 2) RWSEM_READER_OWNED
> + * - lock is currently or previously owned by readers (lock is free
> + * or not set by owner yet)
> + * 3) Other non-zero value
> + * - a writer owns the lock
> + */
> +#define RWSEM_READER_OWNED 1UL
#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
> +
> #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> static inline void rwsem_set_owner(struct rw_semaphore *sem)
> {
> @@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore
> *sem)
> sem->owner = NULL;
> }
>
> +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> +{
> + /*
> + * We check the owner value first to make sure that we will only
> + * do a write to the rwsem cacheline when it is really necessary
> + * to minimize cacheline contention.
> + */
> + if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED)
> + sem->owner = (struct task_struct *)RWSEM_READER_OWNED;
How much if anything did this optimization matter?
> +}
> +
> +static inline bool rwsem_is_writer_owned(struct task_struct *owner)
> +{
> + return (unsigned long)owner > RWSEM_READER_OWNED;
> +}
Tad too clever that; what does GCC generate if you write the obvious:
return owner && owner != RWSEM_READER_OWNER;
> +
> +static inline bool rwsem_is_reader_owned(struct task_struct *owner)
> +{
> + return owner == (struct task_struct *)RWSEM_READER_OWNED;
> +}
So I don't particularly like these names; they read like they take a
rwsem as argument, but they don't.
Would something like: rwsem_owner_is_{reader,writer}() make more sense?
> #else
> static inline void rwsem_set_owner(struct rw_semaphore *sem)
> {
