linux-kernel - Re: [PATCH 1/2] serial: samsung: Move uart_register

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Message-ID: <20140121094529.GQ15937@n2100.arm.linux.org.uk>
Date:	Tue, 21 Jan 2014 09:45:29 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	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 1/2] serial: samsung: Move uart_register_driver call to
	device probe

On Tue, Jan 21, 2014 at 09:25:31AM +0000, One Thousand Gnomes wrote:
> > static DEFINE_MUTEX(foo_mutex);
> > static unsigned foo_devices;
> > 
> > static int foo_probe(struct platform_device *pdev)
> > {
> > 	int ret;
> > 
> > 	mutex_lock(&foo_mutex);
> > 	if (foo_devices++ == 0)
> > 		uart_register_driver(&driver);
> > 
> > 	ret = foo_really_probe_device(pdev);
> 
> We have atomic_inc_and_test and atomic_dec_and_test so it's
> fractionally less ugly.

How do atomics help here?  If we do this as:

	if (atomic_inc_and_test(&foo_atomic))
		uart_register_driver(&driver);

Then let's think about what can happen:

	CPU0				CPU1
	foo_probe
	atomic_inc_and_test() == true
		uart_register_driver
					foo_probe
					atomic_inc_and_test()
					really_probe_foo()
						*bang*

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Estimate before purchase was "up to 13.2Mbit".
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