linux-kernel - Re: [PATCH 1/1] TTY: don't allow reopen when ldisc is changing

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Date:	Sat, 27 Nov 2010 16:11:06 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	unlisted-recipients:; (no To-header on input)
CC:	Kyle McMartin <kyle@...artin.ca>, gregkh@...e.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 1/1] TTY: don't allow reopen when ldisc is changing

On 11/27/2010 10:43 AM, Jiri Slaby wrote:
> On 11/27/2010 09:50 AM, Jiri Slaby wrote:
>> On 11/27/2010 03:59 AM, Kyle McMartin wrote:
>>> I'm poking around to see, I think maybe something might be dropping
>>> locks in the callchain that gives us a window where this might be
>>> possible...
>>
>> Of course, that's the case:
>>         clear_bit(TTY_LDISC, &tty->flags);
>>         tty_unlock();
>>         cancel_delayed_work_sync(&tty->buf.work);
>>         mutex_unlock(&tty->ldisc_mutex);
>>
>>         tty_lock();
>>         mutex_lock(&tty->ldisc_mutex);
>>
>> in tty_ldisc_hangup. Hence my point 1) from previous posts doesn't hold too:
>> 1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
>> section tty_lock is held.
>>
>> I will check, how to fix this.
> 
> Reproducible with 2 running processes from the attachment.

Is it fixed with the attached proof-of-concept patch?

So you need:
THIS ONE
TTY: don't allow reopen when ldisc is changing
TTY: ldisc, fix open flag handling
Char: TTY, restore tty_ldisc_wait_idle

The last one is in 2.6.37-rc2 already.

thanks,
-- 
js
suse labs

View attachment "0001-TTY-open-hangup-race-fixup.patch" of type "text/x-patch" (2329 bytes)