cdev destroy_dev() and detach races

[Kostik Belousov]

On Mon, Jul 02, 2007 at 06:20:58PM +0200, Daniel Roethlisberger wrote:
> Kostik Belousov <kostikbel at gmail.com> 2007-07-02:
> > On Mon, Jul 02, 2007 at 02:01:31PM +0200, Daniel Roethlisberger wrote:
> > > I have been doing some finishing work on my cmx driver [1] and have run
> > > into some difficulties involving detach races.  This is a pccard driver
> > > with a cdev interface, the device is a PCMCIA based smartcard reader.
> > > 
> > > Originally, I thought destroy_dev() would block until all threads have
> > > left the cdevsw handlers.  Then I found out that it would only do so if
> > > d_purge is set in cdevsw.  However, there aren't an awful lot of drivers
> > > using this.  My attempts to use a d_purge handler resulted in crashes
> > > within kern/kern_conf.c when having an endless cat loop running while
> > > detaching the device.
> > Do you referring to RELENG_6 or CURRENT ? In CURRENT, destroy_devl() does
> > sleep until there is any thread that grabbed thread reference. Look at the
> > rev. 1.199 of kern_conf.c. On the other hand, this was never MFCed, and
> > cause some other troubles that are being worked on.
> 
> RELENG_6, sorry for not mentioning.
> 
> > > The solution I am using now is that I sleep in detach() until
> > > cdev->si_threadcount is 0.  This seems to have done the trick, but I
> > > still have to do more testing to be sure.
> > This looks like a workaround.
> 
> Why don't all the other drivers in RELENG_6 need some kind of workaround
> to detach reliably?  Did I just look at the wrong drivers?
The problem you described is a bug in RELENG_6. The fix changes devfs
infrastructure instead of modyfing all the drivers.

One of the reason why the fix (rev. 1.199 of kern_conf.c) it is not
MFCed is some undesirable consequences, in particular, interaction with
drivers calling destroy_dev() from cdev methods.
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