linux-kernel - Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from

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Date:	Tue, 17 Jun 2014 13:31:38 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	David Laight <David.Laight@...lab.com>
Cc:	'Peter Hurley' <peter@...leysoftware.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Karsten Keil <isdn@...ux-pingi.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()

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
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