Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Mon, 24 Aug 2009, Linus Torvalds wrote:
>> 
>> But I wanted to let people know that the patch is clearly not the "last 
>> word" on this. It's a useful thing to try, but we need something better.
>
> This may be better (this is a replacement for the previous patch).
>
> Instead of using 'cancel_delayed_work_sync()', it makes tty_ldisc_hangup() 
> do a 'flush_scheduled_work()' afterwards, like the other callers already 
> do.
>
> And like 'tty_ldisc_release()' already does, it does this all before even 
> getting the ldisc_mutex, avoiding the deadlock.
>
> I'm not 100% happy with this patch either, but my remaining unhappiness is 
> more with the tty locking in general that causes this all. I suspect this 
> patch in itself is not any worse than the other hacks we have.
>
> Oh, and in case you didn't guess - this is _STILL_ totally untested. It 
> compiles for me, but that's all I'm going to guarantee. I'm just looking 
> at the code (and getting pretty fed up with it ;)
>
> And as already mentioned: I doubt the deadlock on tty->ldisc_mutex is 
> anything that would be hit in practice. And even if it can be triggered, 
> the previous patch I sent out is still interesting in a "does it make the 
> problem go away" sense. Because if it doesn't (with or without a new 
> deadlock), then I'm looking at all the wrong places.

Assuming no one beats me to it I should be able to test this tomorrow.

Eric
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