Date: Fri, 24 Sep 2004 11:01:05 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: arch@freebsd.org Subject: I'm counting my threads, one, two, three, four, five... [1] Message-ID: <41458.1096016465@critter.freebsd.dk>
next in thread | raw e-mail | index | archive | help
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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?41458.1096016465>