Re: [PATCH]: disassociate tty locking fixups

[Prarit Bhargava <prarit@redhat.com>]

Alan Cox wrote:

>Ugly but I don't think the patches are sufficient. Firstly you need to
>hold the task lock if you are poking around some other users ->signal,
>or that may itself change. (disassociate_ctty seems to have this wrong)

Ah -- okay.

So the locking order is (for example):

mutex_lock(&tty_mutex);
read_lock(&tasklist_lock);
task_lock(current);

Correct?

>Secondly you appear to have lock ordering issues (you lock tty_mutex in
>both orders relative to the task list lock) (you take tty_mutex first,
>then the task lock which is correct, but then you drop and retake the
>tty_mutex while holding the task lock, which may deadlock)

Fixed.

>Can you also explain why the ctty change proposed is neccessary ?

disassociate_ctty can call tty_vhangup which calls do tty_hangup directly.
do_tty_hangup can then set p->signal->tty = NULL, and after returning to
disassociate_ctty, the value of tty will now contain a bad pointer.  (I can
reproduce this behaviour by running the gdb testsuite with slab debug on)

>NAK the actual code, provisionally agree with the basic diagnosis of
>insufficient locking.

Arjan wrote:

>in addition, are you sure you don't need to revalidate anything after
>retaking the lock?

The only place I need to revalidate data (AFAICT) is in 
disassociate_ctty where
I re-check to see if current->signal->tty is still valid.  Admittedly, I am
looking at a very specific failure path though.

I'll rework the patch and post later.

P.

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