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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1725D868@AcuExch.aculab.com>
Date:	Tue, 17 Jun 2014 08:18:03 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Arnd Bergmann' <arnd@...db.de>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Karsten Keil <isdn@...ux-pingi.de>,
	Peter Hurley <peter@...leysoftware.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: RE: [PATCH tty-next 14/22] tty: Remove
 tty_wait_until_sent_from_close()

From: Arnd Bergmann
> On Monday 16 June 2014 09:17:11 Peter Hurley wrote:
> > tty_wait_until_sent_from_close() drops the tty lock while waiting
> > for the tty driver to finish sending previously accepted data (ie.,
> > data remaining in its write buffer and transmit fifo).
> >
> > However, dropping the tty lock is a hold-over from when the tty
> > lock was system-wide; ie., one lock for all ttys.
> >
> > Since commit 89c8d91e31f267703e365593f6bfebb9f6d2ad01,
> > 'tty: localise the lock', dropping the tty lock has not been necessary.
> >
> > CC: Karsten Keil <isdn@...ux-pingi.de>
> > CC: linuxppc-dev@...ts.ozlabs.org
> > Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> 
> 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.

While I've not looked at the code in question; my thoughts were that
holding any lock while waiting for output to drain (or anything else
really) probably isn't a good idea.
You might find that something else needs the lock - even if only
some kind of status request.

	David