linux-kernel - Re: [PATCH 2/2] serial: pl011: Move uart_register

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Message-ID: <20140214003834.GH30257@n2100.arm.linux.org.uk>
Date:	Fri, 14 Feb 2014 00:38:34 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Tushar Behera <tushar.behera@...aro.org>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, jslaby@...e.cz,
	ben.dooks@...ethink.co.uk, broonie@...nel.org
Subject: Re: [PATCH 2/2] serial: pl011: Move uart_register_driver call to
	device probe

On Thu, Feb 13, 2014 at 04:14:36PM -0800, Greg KH wrote:
> On Fri, Feb 14, 2014 at 12:07:17AM +0000, Russell King - ARM Linux wrote:
> > On Thu, Feb 13, 2014 at 03:26:06PM -0800, Greg KH wrote:
> > > On Thu, Feb 13, 2014 at 06:42:49PM +0000, Russell King - ARM Linux wrote:
> > > > We went through this before, and I stated the paths, and no one disagreed
> > > > with that.
> > > > 
> > > > It /is/ racy.
> > > 
> > > Ok, I just went and looked at the uart driver register path, and I don't
> > > see the race (note, if there is one, it's there today, regardless of
> > > this patch).
> > 
> > The race isn't the uart code, it's the driver model.
> > 
> > Consider what happens when this happens:
> > 
> > * Two pl011 devices get registered at the same time by two different
> >   threads.
> 
> How?  What two different busses will see this same device?  The amba bus
> code should prevent that from happening, right?  If not, there's bigger
> problems in that bus code :)

Where is that requirement documented?  It isn't documented.  No one
implements any kind of locking at the bus level to prevent this, not
PCI, nor platform devices.

> That's where this problem should be fixed, if there is one, otherwise
> this same issue would be there for any type of driver that calles into
> the uart core, right?

And how does bus code prevent this?  By intercepting the driver model
->probe callback and taking its own lock maybe?   Really?  What about
those buses which don't wrap the probe callback?

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