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

[Peter Zijlstra]

On Thu, Nov 11, 2021 at 03:45:30PM -0500, Waiman Long wrote:
> 
> 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.

RWSEM_FLAG_QUIESCE ?