Messages in this thread | | | Date | Thu, 11 Nov 2021 15:45:30 -0500 | Subject | Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already set | From | Waiman Long <> |
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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
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