linux-kernel - Re: [PATCH] tty: avoid recursive BTM in pty

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Message-Id: <201006182035.17394.arnd@arndb.de>
Date:	Fri, 18 Jun 2010 20:35:17 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Tony Luck <tony.luck@...el.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Kacur <jkacur@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] tty: avoid recursive BTM in pty_close

On Friday 18 June 2010, Tony Luck wrote:
> On Fri, Jun 18, 2010 at 9:21 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > So I ack this patch - it's the only way to find out 8)
> 
> Here's the results from this patch on top of next-20100618
> 
> The "WARNING: at include/linux/tty.h:589 tty_open+0x9d0/0xc60()" messages
> are back, two of them, both with same stack trace:
> 
> 
> Call Trace:
>  [<a0000001000159d0>] show_stack+0x50/0xa0
>  [<a00000010090f270>] dump_stack+0x30/0x50
>  [<a00000010008e2c0>] warn_slowpath_common+0xc0/0x120
>  [<a00000010008e360>] warn_slowpath_null+0x40/0x60
>  [<a00000010053eb40>] tty_open+0x160/0xc60
>  [<a0000001001af9b0>] chrdev_open+0x310/0x360
>  [<a0000001001a58b0>] __dentry_open+0x350/0x680
>  [<a0000001001a5d80>] nameidata_to_filp+0x80/0xc0
>  [<a0000001001bfee0>] finish_open+0x160/0x380
>  [<a0000001001c0cc0>] do_last+0xbc0/0xce0
>  [<a0000001001c5270>] do_filp_open+0x2f0/0xb40
>  [<a0000001001a5290>] do_sys_open+0x90/0x200
>  [<a0000001001a54d0>] sys_open+0x50/0x80
>  [<a000000100b907e0>] kernel_init+0x340/0x420
>  [<a000000100013c10>] kernel_thread_helper+0x30/0x60
>  [<a00000010000a0c0>] start_kernel_thread+0x20/0x40

Ok, this is the same one you reported at first. I forgot to
mention that the other patch I sent as a reply to your report
is still needed and not yet in -next since I'm trying to
sort through the other BKL removal patches now.
This instance of the WARN_ON is completely harmless though,
you could consider this one a false positive.

> The tty_release() ones have all gone though.

ok, good.

> I've also just noticed that the serial console output gets garbled
> (like it is at the wrong baud rate) when userland code starts
> printing messages. Stays garbled until /sbin/agetty starts up
> to print the "Welcome ... login:" banner.
>
> Similar garbage when shutting down with "reboot" command.
> I can see "INIT", then it all goes to pieces until I see the kernel
> print "Restarting system".

Do you know if this was a problem with the original series or
something that got introduced by my last patch?
Also, if you don't mind, could you try if the problem also exists
with CONFIG_TTY_MUTEX enabled?
Which serial driver do you use?

I'll try to set up a serial console here to reproduce and bisect
this problem on my own system.

	Arnd
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