linux-kernel - Re: [PATCH]: disassociate tty locking fixups

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Message-ID: <4530FD0F.4050305@redhat.com>
Date:	Sat, 14 Oct 2006 11:06:55 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org, akpm@...l.org, arjan@...radead.org
Subject: Re: [PATCH]: disassociate tty locking fixups

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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