linux-kernel - Re: PROBLEM: Race condition in tty buffer's function flush_to

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Message-ID: <4EB7E0C6.7050909@ilyx.ru>
Date:	Mon, 07 Nov 2011 17:44:38 +0400
From:	Ilya Zykov <ilya@...x.ru>
To:	Alan Cox <alan@...ux.intel.com>
CC:	Greg Kroah-Hartman <gregkh@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: Race condition in tty buffer's function flush_to_ldisc().

Alan Cox wrote:

> 
>> Why we need: "if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) {"
>> if flush_to_ldisc is single threaded?
>> we can: set_bit(TTY_FLUSHING, &tty->flags)
>> without if() at all.
> 
> It is single threaded with respect to itself (you can't have two
> flush_to_ldisc on the same tty at once) but you can have a parallel
> call to tty_buffer_flush. The tty_buffer_flush path needs to pick the
> right approach reliably.
> 

Of course I know about tty_buffer_flush(), it only read TTY_FLUSHING,
it can't change TTY_FLUSHING, if flush_to_ldisc() single threaded,
we can change TTY_FLUSHING only in one place in one time(in flush_to_ldisc()),
therefor we can use only "set_bit(TTY_FLUSHING, &tty->flags)" without test.

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