Re: [PATCH-tip v2 03/12] locking/rwsem: Remove rwsem_wake() wakeup optimization

[Davidlohr Bueso]

On Fri, 05 Apr 2019, Waiman Long wrote:

> With the commit 59aabfc7e959 ("locking/rwsem: Reduce spinlock contention
> in wakeup after up_read()/up_write()"), the rwsem_wake() forgoes doing
> a wakeup if the wait_lock cannot be directly acquired and an optimistic
> spinning locker is present.  This can help performance by avoiding
> spinning on the wait_lock when it is contended.
>
> With the later commit 133e89ef5ef3 ("locking/rwsem: Enable lockless
> waiter wakeup(s)"), the performance advantage of the above optimization
> diminishes as the average wait_lock hold time become much shorter.
>
> By supporting rwsem lock handoff, we can no longer relies on the fact
> that the presence of an optimistic spinning locker will ensure that the
> lock will be acquired by a task soon. This can lead to missed wakeup
> and application hang. So the commit 59aabfc7e959 ("locking/rwsem:
> Reduce spinlock contention in wakeup after up_read()/up_write()")
> will have to be reverted.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> kernel/locking/rwsem-xadd.c | 74 -------------------------------------

I very much like removing this code. It was rather crazy.