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    SubjectRe: [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@openvz.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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