From 5c1ec49b60cdb31e51010f8a647f3189b774bddf Mon Sep 17 00:00:00 2001
From: Waiman Long <longman@redhat.com>
Date: Mon, 20 May 2019 16:59:01 -0400
Subject: [PATCH] locking/rwsem: Remove rwsem_wake() wakeup optimization
After the following 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.
With a later patch that supports 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 and
rwsem_wake() will be called later on to wake up waiters. This can lead
to missed wakeup and application hang.
So the original 59aabfc7e959 commit has to be reverted.