Messages in this thread | | | Date | Mon, 24 Aug 2009 21:10:38 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 |
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On Tue, 25 Aug 2009, Frederic Weisbecker wrote: > > Now that also makes the TTY_LDISC flag clearing unprotected by > tty->ldisc_mutex.
Yes.
> tty_set_ldisc() can play concurrently with these flags right?
.. but that shouldn't matter.
The actual bit-setting is "atomic" already - and any other atomicity is pretty much unattainable, because all the routines in question drop the lock they need to hold in order to make it really be reliably atomic.
> tty_ldisc_halt() could remain protected by the mutex, so that the > flag is safely toggled. Once it is cleared, we can ensure no more > user can ref it and the lock can be relaxed while the pending > work is flushed.
That would make no difference at all. tty_set_ldisc() won't care about the flag (in fact, it will do its own tty_ldisc_halt()), and will be happy to replace the ldisc we just flushed with a new one regardless of whether it was halted before or not. And it will do tty_ldisc_enable() regardless of whether it was enabled or not before.
In fact, because tty_set_ldisc() itself had to release the ldisc_mutex (for the same reason), you have this issue regardless of whether you hold the lock in tty_hangup() or not: the two will always be able to get "mixed up", because they - by design - have to release that silly lock.
That's why I said I was unhappy about the tty layer locking - it really isn't very sane. Things like tty_set_ldisc() will drop the lock in the middle because of that crazy workqueue deadlock - exactly for the same reasons that tty_ldisc_hangup() will need to do that "wait for things to flush" without the lock held.
So I could have taken the ldisc_mutex, and then just dropped it temporarily while waiting for any workqueue entries, but as far as I can tell, it doesn't actually solve anything.
I considered using the TTY_LDISC_CHANGING bit(*) there to protect against tty_set_ldisc(), and it may even be the right solution. But there's no way I'll do that kind of changes this late in the -rc series.
We also have the "TTY_HUPPED" bit that disables tty_set_ldisc(), but that is set too late by do_tty_hangup(), and so doesn't fix the problem either. Again, moving it earlier may be a solution, but again, it's not appropriate for this late in the -rc.
Finally, the solution that is most likely the _real_ solution would be to just fix the locking. The whole "ldisc_mutex" seems dubious. It's not even a real lock - exactly because it's dropped - and we already really use that TTY_LDISC_CHANGING bit to do the _real_ locking. I don't think it needs to be a mutex at all. The locking is just very dubious.
And that, least of all, is anything I'm willing to really do in -rc.
Anyway, I'll happily be shown wrong. I think the (second) patch I sent out is an acceptable hack in the presense of the current locking, but as I said, I'm not exactly happy about it, because I do think the locking is broken.
Linus
(*) We already have that hacky open-coded "lock" using TTY_LDISC_CHANGING, which protects two different tty_set_ldisc()'s from screwing up each other when they drop the semaphore. It could be just separated out into a function of its own, and then the hangup code would/could/should be taught to use that logic.
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