Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already set

[Waiman Long <longman@redhat.com>]

On 11/11/21 15:35, Peter Zijlstra wrote:
> On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
>> On 11/11/21 14:20, Peter Zijlstra wrote:
>>> On Thu, Nov 11, 2021 at 02:14:48PM -0500, Waiman Long wrote:
>>>> As for the PHASE_CHANGE name, we have to be consistent in both rwsem and
>>>> mutex. Maybe a follow up patch if you think we should change the
>>>> terminology.
>>> Well, that's exactly the point, they do radically different things.
>>> Having the same name for two different things is confusing.
>>>
>>> Anyway, let me go read that patch you sent.
>> My understanding of handoff is to disable optimistic spinning to let waiters
>> in the wait queue have an opportunity to acquire the lock. There are
>> difference in details on how to do that in mutex and rwsem, though.
> Ah, but the mutex does an actual hand-off, it hands the lock to a
> specific waiting task. That is, unlock() sets owner, as opposed to
> trylock().
>
> The rwsem code doesn't, it just forces a phase change. Once a waiter has
> been blocked too long, the handoff bit is set, causing new readers to be
> blocked. Then we wait for existing readers to complete. At that point,
> any next waiter (most likely a writer) should really get the lock (and
> in that regards the rwsem code is a bit funny).
>
> So while both ensure fairness, the means of doing so is quite different.
> One hands the lock ownership to a specific waiter, the other arranges
> for a quiescent state such that the next waiter can proceed.

That is a valid argument. However, the name PHASE_CHANGE sounds weird to 
me. I am not objecting to changing the term, but probably with a better 
name NO_OPTSPIN, NO_LOCKSTEALING or something like that to emphasize 
that fact that optimistic spinning or lock stealing should not be allowed.

Anyway, it will be a follow-up patch.

Cheers,
Longman