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

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Date:	Wed, 09 Jul 2014 09:57:50 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Laight <David.Laight@...LAB.COM>,
	Arnd Bergmann <arnd@...db.de>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
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>
Subject: Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()

On 06/17/2014 07:32 AM, Peter Hurley wrote:
> 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.

Further, the tty lock should not be nested within the tty_mutex lock
in a reopen, regardless of O_NONBLOCK.

AFAICT, the tty_mutex in the reopen scenario is only protecting the
tty count bump of the linked tty (if the tty is a pty).

I think with some refactoring and returning with a tty reference held
from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty
lock in tty_open() can be attempted without nesting in the tty_mutex.

Regardless, I'll be splitting this series and I'll be sure to cc
you all when I resubmit these changes (after testing).

Regards,
Peter Hurley




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