I'm counting my threads, one, two, three, four, five... [1]

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To: arch@freebsd.org
Date: Fri, 24 Sep 2004 11:01:05 +0200

I've completed my sweep of the kernel and I belive that we now have a
reliable count of the number of threads currently inside any particular
cdev and consequently, with a bit of adding up, also for each cdevsw.

There are two soft spots marked with XXX (devfs_rule and vm_mmap)
and one big hack, snp(4).

The next step is to add a new method to the cdevsw which purges threads
from the driver. The best name I can come up with is d_evict(). The
drivers should make this function wakeup(9) on anything a thread could
be sleeping on for the relevant cdev and the tread on waking up should
examine whatever bits it can, realize the hw/driver/whatever is gone
and exit the driver with ENXIO error return.

If this cdevsw method is present, destroy_dev() will use it and will
sleep until all threads have cleared out.

So for a typical driver things will look like this:

foo_evict(struct cdev *dev)
{
        struct foo_sc *sc;

        /* XXX: NB: no locks, only wakeups */
        sc = dev->si_drv1;
        wakeup(sc->bing);
        wakeup(sc->bongle);
        wakeup(sc->bang);
}

foo_detach(dev)
{
        ...

        sc->flags |= GONE;
        destroy_dev(sc->cdev);
        /* XXX: we now know there are no threads involved with this cdev */
        ...
}

destroy_dev() will work more or less this way:

        ...
        dev_lock();
        csw = cdev->si_devsw;
        cdev->si_devsw = NULL
        while (csw->d_evict != NULL && cdev->si_threacount > 0) {
                (csw->d_evict)(cdev);
                msleep(&dev_lock, ..., hz/10);
        }
        dev_unlock();
        ...

I will also add a convenience function called destroy_cdevsw() which
will call destroy_dev() on all cdevs using the cdevsw in question.

I belive this gives us the handle we need to unload drivers and remove
hardware without panicing in the lower layers of the kernel. The
higher layers may still have a thing or two to learn in this respect.

Poul-Henning

[1] It used to be possible to legally download the D:A:D tune which
inspired the subject, but I can't find the link anymore, sorry :-(

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
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