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

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Message-ID: <20111107183950.4ec7a959@lxorguk.ukuu.org.uk>
Date:	Mon, 7 Nov 2011 18:39:50 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ilya Zykov <ilya@...x.ru>
Cc:	Alan Cox <alan@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: Race condition in tty buffer's function
 flush_to_ldisc().

> Nested call flush_to_ldisc() happened on different CPU only. It happens because
> one side pty call "schedule_work(&tty->buf.work)" on one CPU from "tty_flip_buffer_push()",
> the other side call "schedule_work(&tty->buf.work)" from n_tty's layer from "n_tty_set_room()"
> on different CPU. It also can happen in interrupt if interrupt handle on 
> different CPU. Schedule_work() schedule work on CPU which was called(IMHO).

But the two sides are different ttys so that specific case shouldn't
matter.

Something is clearly happening because you trigger the trace but I don't
see why the paths you indicate cause it ? If you make mthread a cpu
struct variable and add a WARN_ON(1) in the error case do you get any
useful traces ?

Alan
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