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

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Date:	Fri, 01 Jun 2012 20:17:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] tty: add lockdep annotations

On Thu, 2012-05-31 at 11:35 +0200, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> tty_lock_pair() do the right thing to avoid deadlocks, but should
> instruct LOCKDEP of this to avoid a splat. 
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
> Latest Linus tree still hang on my machine, at least this patch removes
> the lockdep problem...

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

Call Trace:

	con_shutdown+0x3e/0x50
	queue_release_one_tty+0x24/0x70
	tty_kref_put+0x28/0x30
	tty_unlock+0x2a/0x60
	tty_open+0x4d8/0x620
	chrdev_open+0xb1/0x1a0
...

Sometime stack is :
memset()
alloc_tty_struct
pty_unix98_install
tty_init_dev
ptmx_open
chrdev_open


I suspect a refcounting problem, but am not familiar enough with tty
code....



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