From: Arnd Bergmann <arnd@arndb.de>
To: David Laight <David.Laight@aculab.com>
Cc: "'Peter Hurley'" <peter@hurleysoftware.com>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Karsten Keil <isdn@linux-pingi.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()
Date: Tue, 17 Jun 2014 13:31:38 +0200
Message-ID: <8191805.dfI4LbYmWz@wuerfel>
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References: <1402924639-5164-1-git-send-email-peter@hurleysoftware.com> <53A01F02.7000202@hurleysoftware.com> <063D6719AE5E284EB5DD2968C1650D6D1725DAF6@AcuExch.aculab.com>
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On Tuesday 17 June 2014 11:03:50 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.

Before the patch, I believe tty_reopen() would return -EIO because
the TTY_CLOSING flag is set. After the patch, tty_open() blocks
on tty_lock() before calling tty_reopen(). AFAICT, this is independent
of O_NONBLOCK.

	Arnd
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