Date: Fri, 17 May 2013 13:48:45 +0200
From: Joerg Roedel <joro@8bytes.org>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Jiri Slaby <jslaby@suse.cz>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tty: Add missing lock in n_tty_write()
Message-ID: <20130517114845.GA2602@8bytes.org>
References: <20130515105656.GI24440@8bytes.org>
 <5193A3B5.7010400@suse.cz>
 <20130515154753.GJ24440@8bytes.org>
 <5193D7E0.5010803@hurleysoftware.com>
 <20130515194835.GK24440@8bytes.org>
 <519415F3.6060103@hurleysoftware.com>
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Hi Peter,

thanks for you explanations. They helped me to better understand what is
happening now.

On Wed, May 15, 2013 at 07:10:43PM -0400, Peter Hurley wrote:
> On 05/15/2013 03:48 PM, Joerg Roedel wrote:

> Agreed. Those functions look written for single-producer/single-consumer
> i/o model. (That's why I asked about CONFIG_CONSOLE_POLL=y as well because
> that doesn't look thread-safe either).

Ok, I checked that. CONFIG_CONSOLE_POLL is on in that kernel.

> Just to be clear here: there's a difference between a console driver
> and a tty driver.
> 
> The console driver's write() method is serialized with the global
> console_lock() so parallel console writes are not possible.
> 
> No such guarantee exists for the tty driver write() method, although it
> probably wouldn't be difficult to provide that guarantee (since the
> line discipline write() is already serialized by tty->atomic_write_lock).

Okay, so it is safe to say that currently the drivers write() (and
put_chars()) functions need to expect to be called concurrently and
therefore they have to serialize themselves when they need it, right?


	Joerg


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