Messages in this thread | | | Date | Mon, 2 May 2005 23:27:21 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] tty races |
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Jason Baron <jbaron@redhat.com> wrote: > > > On Mon, 25 Apr 2005, Andrew Morton wrote: > > > Jason Baron <jbaron@redhat.com> wrote: > > > > > > There are a couple of tty race conditions, which lead to inconsistent tty > > > reference counting and tty layer oopses. > > > > > > The first is a tty_open vs. tty_close race in drivers/char/tty.io.c. > > > Basically, from the time that the tty->count is deemed to be 1 and that we > > > are going to free it to the time that TTY_CLOSING bit is set, needs to be > > > atomic with respect to the manipulation of tty->count in init_dev(). This > > > atomicity was previously guarded by the BKL. However, this is no longer > > > true with the addition of a down() call in the middle of the > > > release_dev()'s atomic path. So either the down() needs to be moved > > > outside the atomic patch or dropped. I would vote for simply dropping it > > > as i don't see why it is necessary. > > > > The release_dev() changes looks very fishy to me. It _removes_ locking. > > If that fixes the testcase then one of two things is happening: > > > > a) we have lock_kernel() coverage and the down()'s sleeping breaks the > > lock_kenrel() coverage or > > > > b) we don't have lock_kernel() coverage, but removing the down() just > > alters the timing and makes the race less probable. > > > > I think it's b). lock_kernel() coverage in there is very incomplete on the > > open() side. > > > > The patch was written for case a. Indeed lock_kernel() may appear > incomplete on the open side, but it protects paths where we don't sleep. > So, the 'fast_track' path in 'init_dev', is protected against the > release_dev path from setting the 'tty_closing' local variable to the > setting of the TTY_CLOSING flag. Thus, i believe the dropping of the > down() is correct.
I don't see anywhere which takes lock_kernel() on the tty_open() path.
The normal release_dev() path takes lock_kernel(), but two error-path callers of lock_kernel() also appear to not take lock_kernel().
> This was the previous locking model for open vs. close afaict, before the > down() was introduced in the release_dev path that was supposed to be > atomic with respect to init_dev().
We want to move away from lock_kernel()-based locking.
> > > I think it would be better to _increase_ the tty_sem coverage in > > release_dev() and to make sure that all callers of init_dev() are using > > tty_sem (they are). > > > > One approach would be to require that all callers of release_dev() hold > > tty_sem, and make release_dev() drop and reacquire tty_sem in those cases > > where release_dev() needs to go to sleep when waiting for other threads of > > control to reelase the tty's resources. > > > > Indeed, the situation would be improved if it was held around the > driver->close() routine. This routine does sometimes look at tty->count > value, see con_close(), where in fact the tty_sem is added to avoid just > this problem. However, it is incorrect as one can see in release_dev() the > schedule(), can cause the tty->count to change via tty_open(). However, i > think this is an extremely rare corner case, b/c con_close() keys off > tty->count of 1, which implies that this is the last close() and thus the > schedule for 'write_wait' would seem impossible, although AL Viro has > said that it is possible in this case. Thus, dropping the tty_sem and > reacquiring it, probably isn't good, b/c the driver->close() routines can > free resources based upon tty->count==1.
Maybe we can just hold tty_sem across that schedule() in release_dev().
If not, then maybe retest ->count and take avoiding action if it looks like some other thread is trying to resurrect the tty. Obviously this is a much poorer approach.
> The patch was written as the least invasive and low risk way to fix a > nasty race condition, which has the potential to corrupt data. The oops in > vt_ioctl has also been seen on system boots with some frequency. The patch > imo, returns the the tty_open vs. tty_close paths to their original > locking assumptions which have been well tested. >
I don't think it does, and the original lock_kernel-based locking is obsolete.
Please, let's do this properly, with real locks.
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