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

[Peter Zijlstra]

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.