On Fri, Jan 30, 2015 at 01:14:26AM -0800, Davidlohr Bueso wrote:
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -337,21 +337,30 @@ static inline bool owner_running(struct rw_semaphore
> *sem,
> static noinline
> bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
> {
> + long count;
> +
> rcu_read_lock();
> while (owner_running(sem, owner)) {
> + /* abort spinning when need_resched */
> + if (need_resched()) {
> + rcu_read_unlock();
> + return false;
> + }
>
> cpu_relax_lowlatency();
> }
> rcu_read_unlock();
>
> + if (READ_ONCE(sem->owner))
> + return true; /* new owner, continue spinning */
> +
Same concern as Tim; also the mutex code seems to terminate the spin
when owner changes. And I think we want to have writers behave similar
to mutexes, no?
Does it make sense to change things to allow owner changes from NULL,
but not to NULL?
> /*
> + * When the owner is not set, the lock could be free or
> + * held by readers. Check the counter to verify the
> + * state.
> */
> - return sem->owner == NULL;
> + count = READ_ONCE(sem->count);
> + return (count == 0 || count == RWSEM_WAITING_BIAS);
> }
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