Messages in this thread | | | Date | Sun, 16 Oct 2011 21:37:46 +0200 | From | Jiri Slaby <> | Subject | Re: [PATCH 4/4] TTY: call tty_driver_lookup_tty unconditionally |
| |
Fixing Alan's address. Somehow I put there a RH (defunct) one.
On 10/16/2011 09:20 PM, Sukadev Bhattiprolu wrote: > Jiri Slaby [jirislaby@gmail.com] wrote: > | On 10/12/2011 11:32 AM, Jiri Slaby wrote: > | > Commit 4a2b5fddd5 (Move tty lookup/reopen to caller) made the call to > | > tty_driver_lookup_tty conditional in tty_open. It doesn't look like it > | > was an intention. Or if it was, it was not documented in the changelog > | > and the code now looks weird. For example there would be no need to > | > remember the tty driver and tty index. Further the condition depends > | > on a tty which we drop a reference of already. > | > > | > If I'm looking correctly, this should not matter thanks to the locking > | > currently done there. Thus, tty_driver->ttys[idx] cannot change under > | > our hands. But anyway, it makes sense to change that to the old > | > behaviour. > | > | Well, this doesn't work for ptys. devpts lookup code expects an inode to > | be one of devpts filesystem (/dev/pts/*), not something on tmpfs like > | /dev/tty. So it looks like the change was intentional, but very > | undocumented and leaving there some unused code. > > Yes this was intentional - even though the tty_driver_lookup() was > unconditional in tty_init_dev() I believe it did not do anything useful > when called from ptmx_open(). ptmx_open() would be setting up a new pty and > the lookup would not find a tty.
Yes, I'm not arguing against moving the code from tty_init_dev to tty_open change. That is perfectly OK.
What I mind is that now we do: ===== tty = get_current_tty(); if (!tty) return -ENXIO; driver = tty_driver_kref_get(tty->driver); /* ZZZ */ index = tty->index; /* ZZZ */ ... tty_kref_put(tty); /* XXX */ goto got_driver; ... got_driver: if (!tty) { /* YYY */ } =====
But at the point of YYY, the tty may be invalid due to reference drop at XXX. I *hope* it is not the case thanks to the current locking there (namely BTM) but I'm really *not sure*. And if it is the case, there should definitely be a note. (Or better the reference should be held while necessary.)
> Would the following change to tty_open() help ?
No, it doesn't matter now. I would prefer a description of the change to be a part of the commit log. And it missed such a notice.
> I am not sure about the unused code you refer to above. Can you please > clarify ?
It is index and driver assigned in the branch above (see ZZZ). When we have a live tty (which I'm not sure we have -- see above), it is guaranteed that the driver is reff'ed. And we need neither driver nor index when we have such a tty.
thanks, -- js
| |