linux-kernel - Re: [RFC] tty: Add get- ioctls to fetch tty status

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Date:	Thu, 13 Sep 2012 20:41:00 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [RFC] tty: Add get- ioctls to fetch tty status

On Thu, Sep 13, 2012 at 05:25:07PM +0100, Alan Cox wrote:
> On Thu, 13 Sep 2012 16:54:01 +0400
> Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> 
> > On Thu, Sep 13, 2012 at 01:51:31PM +0100, Alan Cox wrote:
> > > > +static int pty_get_lock(struct tty_struct *tty, int __user *arg)
> > > > +{
> > > > +	int locked = test_bit(TTY_PTY_LOCK, &tty->flags);
> > > > +	if (put_user(locked, arg))
> > > > +		return -EFAULT;
> > > 
> > > Now explain exactly how this doesn't race with another thread chanigng
> > > the lock setting ?
> > 
> > It's the same as to set/clear this bit, isn't it? Please correct me
> > if I'm wrong.
> 
> So by the time you've finished the test bit and returned it to user space
> the answer may have changed?

Yes. Btw, as far as I see the same applies to lock bit on its own. One
process may lock it while another process may unlock it right after that.

> > > The other comment I have is that it might be better put these in now
> > > there are sysfs patches for the tty layer bouncing about to provide the
> > > needed infrastructure ?
> > 
> > Alan, could you please point me where these patches are living, so I would
> > take a look and check them out
> 
> linux-serial or check tty-next as I think Greg has now merged the test
> patch.

Ah, thanks a lot, Alan, I'll take a look! 

	Cyrill
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