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

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Message-ID: <53A0273F.70400@hurleysoftware.com>
Date:	Tue, 17 Jun 2014 07:32:15 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Laight <David.Laight@...LAB.COM>,
	Arnd Bergmann <arnd@...db.de>
CC:	"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 06/17/2014 07:03 AM, 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.

Good point.

tty_open() should be trylocking both mutexes anyway in O_NONBLOCK.

Regards,
Peter Hurley
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