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

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Date:	Tue, 17 Jun 2014 11:03:50 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Peter Hurley' <peter@...leysoftware.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:	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()

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.

	David