linux-kernel - Re: [PATCHv4 05/17] pps: access pps device by direct pointer

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Date:	Mon, 22 Nov 2010 17:55:47 +0300
From:	Alexander Gordeev <lasaine@....cs.msu.su>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Rodolfo Giometti <giometti@...eenne.com>,
	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCHv4 05/17] pps: access pps device by direct pointer

В Sun, 21 Nov 2010 14:12:26 +0000
Alan Cox <alan@...rguk.ukuu.org.uk> пишет:

> 
> > Maybe this lock is useless... however, are we sure that before setting
> > tty->disc_data to pps its value is null? Otherwise the dcd_change may
> > be called with an oops! We cannot control serial port IRQ
> > generation... :-/
> 
> tty->disc_data is LDISC private. It may be any arbitary value on entry to
> the ldisc open method.
> 
> 
> > > +	spin_lock_irq(&pps_ldisc_lock);
> > > +	tty->disc_data = NULL;
> > > +	spin_unlock_irq(&pps_ldisc_lock);
> 
> And you don't need to set it to NULL after - some ldiscs do this mostly
> as a debug aid.

It was a part of attempt to do a workaround. But it still is not safe
enough because it doesn't protect from a sudden ldisc change while we
are at the beginning of pps_tty_dcd_change() i.e. haven't acquired spin
lock yet. The real fix is ready, I'll send it soon.

-- 
  Alexander

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