linux-kernel - Re: [PATCH] tty: add lockdep annotations

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Date:	Fri, 1 Jun 2012 21:56:20 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] tty: add lockdep annotations

> Yes, tty->driver deref is ok (tty points to valid memory), but crash is
> on tty->driver->ops (driver points to freed/illegal memory)
> 
> using slub_debug=FZPU, I can indeed see RDI=6b6b6b6b6b6b6b6b

driver and driver->ops is basically const and it's not what you'd expect
from a tty refcount bug. The driver side puts shouldn't have changed but
I'll take a look over that patch and the error paths closely again just
in case.

It could be that tty->driver is pointing at a valid but bogus location
but again its not something I'd expect.

> By the way, release_one_tty() uses the following racy code :
> 
> tty_driver_kref_put(driver);
> module_put(driver->owner);
> 
> I would use following patch to make sure bad things cant happen...

Agreed. Although we wouldn't be unloading console or pty so it's alas not
the real cause.


Alan
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