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

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Message-ID: <CA+55aFzrbmGjghmR3M=fWrGUdy6BDu+SVXz6rjM4QAuUjfxk_w@mail.gmail.com>
Date:	Fri, 1 Jun 2012 11:51:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	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

On Fri, Jun 1, 2012 at 11:17 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> About 10% of boots on my machine, and this looks like (hand written)
>
> general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
> ...
> RIP : tty_shutdown+0x15/)x70

Ok, DEBUG_PAGEALLOC and with that offset fairly early in tty_shutdown,
it's almost certainly one of the accesses in the

  tty->driver->ops->remove

chain when it does the (inlined)

    tty_driver_remove_tty(tty->driver, tty);

you could check which one it is at that offset 0x15, but I think both
the ops and the driver structures should be statically allocated, so I
suspect it's the "tty" itself that is already freed.

Odd. But yes, smells very much like a refcount issue, probably due to
broken locking. Does the problem go away if you revert commits
d29f3ef39be4 ("tty_lock: Localise the lock") and 3af502b96649
("tty_lock: undo the old tty_lock use on the ctty")?

                  Linus
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