Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 17 June 2014 11:03:50 David Laight wrote:
> From: Peter Hurley
> ...
> > > I don't understand the second half of the changelog, it doesn't seem
> > > to fit here: there deadlock that we are trying to avoid here happens
> > > when the *same* tty needs the lock to complete the function that
> > > sends the pending data. I don't think we do still do that any more,
> > > but it doesn't seem related to the tty lock being system-wide or not.
> > 
> > The tty lock is not used in the i/o path; it's purpose is to
> > mutually exclude state changes in open(), close() and hangup().
> > 
> > The commit that added this [1] comments that _other_ ttys may wait
> > for this tty to complete, and comments in the code note that this
> > function should be removed when the system-wide tty mutex was removed
> > (which happened with the commit noted in the changelog).
> 
> What happens if another process tries to do a non-blocking open
> while you are sleeping in close waiting for output to drain?
> 
> Hopefully this returns before that data has drained.

Before the patch, I believe tty_reopen() would return -EIO because
the TTY_CLOSING flag is set. After the patch, tty_open() blocks
on tty_lock() before calling tty_reopen(). AFAICT, this is independent
of O_NONBLOCK.

	Arnd