protection against module unloading ?

[Luigi Rizzo]

On Mon, Jul 13, 2015 at 06:29:12PM +0300, Konstantin Belousov wrote:
> On Mon, Jul 13, 2015 at 05:00:30PM +0200, Luigi Rizzo wrote:
...
> > thanks a lot for the clarification on the intent.
> > I clearly need to understand more on the architecture of the module unload.
> > 
> > In any case: the global contention on devmtx for I/O syscalls is
> > really a showstopper for making effective use of modular drivers
> > so we should really find a way to remove it.
> What contention do you see ?  Is it on the device node for a single
> device ?  If yes, then any modification of the below proposal would
> not help.  I explained this below.

It was adrian that pointed it out to me the huge devmtx contention
with multiple threads doing I/O on netmap file descriptor
(4-8 threads each of them issuing around 200K syscalls/s)

Now i see how even if my idea of per-dev lock was correct
it would not remove contention at all.

One final thing:

> > Is there any other way to protect access to dev->si_threadcount ?
> > 
> > Eg how about the following:
> > - use a (leaf) lock into struct cdev to protect dev->si_threadcount, so that
> >   we could replace dev_lock() with mtx_lock(&dev->foo) in dev_refthread(dev)
> >   dev_relthread(dev) and other places that access si_threadcount
> This would not work, you cannot protect a lifetime of the object by a lock
> contained in the object.

i thought so but then the current dev_refthread() is already unsafe,
accessing dev->si_flags unprotected

    sys/kern/kern_conf.c:

	struct cdevsw *
	dev_refthread(struct cdev *dev, int *ref)
	{
		struct cdevsw *csw;
		struct cdev_priv *cdp;

		mtx_assert(&devmtx, MA_NOTOWNED);
		if ((dev->si_flags & SI_ETERNAL) != 0) {
			*ref = 0;
			return (dev->si_devsw);
		}
		dev_lock();
		csw = dev->si_devsw;
		if (csw != NULL) {
			cdp = cdev2priv(dev);
			if ((cdp->cdp_flags & CDP_SCHED_DTR) == 0)
				atomic_add_long(&dev->si_threadcount, 1);
			else
				csw = NULL;
		}
		dev_unlock();
		*ref = 1;
		return (csw);
	}

that is particularly bad though, because it prevents from
checking the SI_ETERNAL flag without holding the lock (short
of encoding the flag in the pointer!)

cheers
luigi