Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion

[Andreas Gruenbacher]

On Mon, Aug 23, 2021 at 6:06 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Mon, Aug 23, 2021 at 05:18:12PM +0200, Andreas Gruenbacher wrote:
> > On Mon, Aug 23, 2021 at 10:14 AM Steven Whitehouse <swhiteho@xxxxxxxxxx> wrote:
> > > If the goal here is just to allow the glock to be held for a longer
> > > period of time, but with occasional interruptions to prevent
> > > starvation, then we have a potential model for this. There is
> > > cond_resched_lock() which does this for spin locks.
> >
> > This isn't an appropriate model for what I'm trying to achieve here.
> > In the cond_resched case, we know at the time of the cond_resched call
> > whether or not we want to schedule. If we do, we want to drop the spin
> > lock, schedule, and then re-acquire the spin lock. In the case we're
> > looking at here, we want to fault in user pages. There is no way of
> > knowing beforehand if the glock we're currently holding will have to
> > be dropped to achieve that. In fact, it will almost never have to be
> > dropped. But if it does, we need to drop it straight away to allow the
> > conflicting locking request to succeed.
>
> It occurs to me that this is similar to the wound/wait mutexes
> (include/linux/ww_mutex.h & Documentation/locking/ww-mutex-design.rst).
> You want to mark the glock as woundable before faulting, and then discover
> if it was wounded after faulting.  Maybe sharing this terminology will
> aid in understanding?

I've looked at the ww_mutex documentation. A "transaction" wounds
another "transaction" and that other transaction then "dies", or it
"heals" and restarts. In the glock case, a process sets and clears the
HIF_MAY_DEMOTE flag on one of its own glock holder contexts. After
clearing the flag, it either still holds the glock or it doesn't;
nothing needs to be done to "die" or to "heal". So I'm not sure we
want to conflate two concepts.

One of the earlier terms we've used was "stealing", with a
HIF_MAY_STEAL flag. That works, but it's slightly less obvious what
happens to a glock holder when the glock is stolen from it. (The
holder gets dequeued, __gfs2_glock_dq.) The glock code already uses
the terms promote/demote, acquire/release, enqueue/dequeue, and
_nq/_dq for various forms of acquiring and releasing a glock, so we're
not in a shortage or names right now apparently.

Thanks,
Andreas