Re: [PATCH v4 00/30] NT synchronization primitive driver

Peter Zijlstra
2024-04-16T16:19:28Z

On Tue, Apr 16, 2024 at 05:53:45PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 16, 2024 at 05:50:14PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 16, 2024 at 10:14:21AM +0200, Peter Zijlstra wrote:
> >
> > > > Some aspects of the implementation may deserve particular comment:
> > > >
> > > > * In the interest of performance, each object is governed only by a single
> > > > spinlock. However, NTSYNC_IOC_WAIT_ALL requires that the state of multiple
> > > > objects be changed as a single atomic operation. In order to achieve this, we
> > > > first take a device-wide lock ("wait_all_lock") any time we are going to lock
> > > > more than one object at a time.
> > > >
> > > > The maximum number of objects that can be used in a vectored wait, and
> > > > therefore the maximum that can be locked simultaneously, is 64. This number is
> > > > NT's own limit.
> >
> > AFAICT:
> >
> > spin_lock(&dev->wait_all_lock);
> > list_for_each_entry(entry, &obj->all_waiters, node)
> > for (i=0; i<count; i++)
> > spin_lock_nest_lock(q->entries[i].obj->lock, &dev->wait_all_lock);
> >
> > Where @count <= NTSYNC_MAX_WAIT_COUNT.
> >
> > So while this nests at most 65 spinlocks, there is no actual bound on
> > the amount of nested lock sections in total. That is, all_waiters list
> > can be grown without limits.
> >
> > Can we pretty please make wait_all_lock a mutex ?
>
> Hurmph, it's worse, you do that list walk while holding some obj->lock
> spinlokc too. Still need to figure out how all that works....

So the point of having that other lock around is so that things like:

try_wake_all_obj(dev, sem)
try_wake_any_sem(sem)

are done under the same lock?

Where I seem to note that both those functions do that same list
iteration.

Can't you write things like:

static void try_wake_all_obj(struct nysync_device *dev,
struct ntsync_obj *obj,
void (*wake_obj)(struct ntsync_obj *obj))
{
list_for_each_entry(entry, &obj->all_waiters, node) {
spin_lock(&obj->lock);
try_wake_all(dev, event->q, obj);
wake_obj(obj);
spin_unlock(&obj->lock);
}
}

And then instead of the above, write:

try_wake_all_obj(dev, sem, wake_sem);

[[ Also, should not something like try_wake_any_sem -- wake_sem in the
above -- have something like:

WARN_ON_ONCE(sem->type != NTSYNC_TYPE_SEM);
]]