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Date: Tue, 17 Jun 2014 07:32:15 -0400
From: Peter Hurley <peter@hurleysoftware.com>
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To: David Laight <David.Laight@ACULAB.COM>, Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH tty-next 14/22] tty: Remove
 tty_wait_until_sent_from_close()
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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