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

[Waiman Long]

On 11/11/21 16:27, Peter Zijlstra wrote:
> 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 ?

I think that is a more acceptable term than PHASE_CHANGE. Will have a 
follow up patch later on. This one is more urgent and I want to get it 
done first.

Cheers,
Longman.