Date: Thu, 2 Aug 2012 00:02:10 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Ed Schouten <ed@80386.nl> Cc: current@freebsd.org, hselasky@freebsd.org Subject: Re: ttydev_cdevsw has no d_purge Message-ID: <20120801210210.GU2676@deviant.kiev.zoral.com.ua> In-Reply-To: <CAJOYFBDsK8cZYc28sKcC0qcZrpy2=A3QAHvP5fEj9gn=Acwciw@mail.gmail.com> References: <20120801160323.GN2676@deviant.kiev.zoral.com.ua> <CAJOYFBDsK8cZYc28sKcC0qcZrpy2=A3QAHvP5fEj9gn=Acwciw@mail.gmail.com>
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--ilwtao6+P+Teod98 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Aug 01, 2012 at 10:46:58PM +0200, Ed Schouten wrote: > Hi Kostik, >=20 > 2012/8/1 Konstantin Belousov <kostikbel@gmail.com>: > > I would blame tty subsystem rather then USB subsystem. The d_purge > > method of the ttydev_cdevsw is not implemented, but it is the only > > measure that can break the deadlocks like the one I described. The > > d_purge shall wake up threads sleeping inside devsw methods, which was > > specifically added to notify driver about device gone events. >=20 > I guess d_purge was added quite recently, right? No, it was there at least in 2006. In fact, it seems to be added in 2004, see r135843. >=20 > In the current design, the USB code must call into tty_gone() to You mean tty_rel_gone(), right ? > report that the TTY is no longer associated with an actual device. > This shall result in all threads blocking on a TTY to be woken up. > Also, it will prevent any further calls into the USB code by the TTY > layer. >=20 > Still, if the processes are not actually interacting with the TTY > (e.g. sleep 100000, just waiting for nanosleep() to return), then > there is no way the TTY code can actually garbage collect the TTY. It > must stay there. Removing the actual TTY would introduce a whole bunch > of races and unwanted behaviour. Applications may cache the pathname > to the TTY. If the pathname were to be reused by another device, apps > would start writing to random TTYs. So that's why TTYs may still stick > around in devfs, even though the device underneath went missing. The > driver simply calls tty_gone() and leaves the TTY alone. It will die > eventually, but you shouldn't wait for it to happen. Well, IMO it is weird, and tty should be destroyed immediately. Do we have any problems with pts-style pty which would force reuse of device names ? For hardware ttys, immediate removal looks just right unconditionally. >=20 > Still, I seem to remember the USB code does something weird. I think > the USB serial driver tries to block until the TTY is actually > destroyed. This is a bug, which I've discussed with hselasky@ numerous > times. It simply must not do that. Yes, it does. ucom_detach_tty(): tty_rel_gone(tp); mtx_lock(sc->sc_mtx); /* Wait for the callback after the TTY is torn down */ while (sc->sc_ttyfreed =3D=3D 0) cv_wait(&sc->sc_cv, sc->sc_mtx); and sc_ttyfreed is set from destroy_dev_sched callback. --ilwtao6+P+Teod98 Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlAZmVIACgkQC3+MBN1Mb4g/bgCg8E/RDnksdoqCy4SZF1CEdDkC KbkAoOZ5UeKLTqcn4cZ1EtEwl7GRKYxa =uxK2 -----END PGP SIGNATURE----- --ilwtao6+P+Teod98--
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