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Date:   Wed, 10 Apr 2019 11:38:13 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH-tip v2 03/12] locking/rwsem: Remove rwsem_wake() wakeup
 optimization
Message-ID: <20190410183813.n4sybugomkn53gvu@linux-r8p5>
References: <20190405192115.17416-1-longman@redhat.com>
 <20190405192115.17416-4-longman@redhat.com>
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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@redhat.com>
>---
> kernel/locking/rwsem-xadd.c | 74 -------------------------------------

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