linux-kernel - Re: Locking in the clk API

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Message-ID: <20110127210729.79eef2c1@lxorguk.ukuu.org.uk>
Date:	Thu, 27 Jan 2011 21:07:29 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh <linux-sh@...r.kernel.org>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Richard Zhao <linuxzsc@...il.com>
Subject: Re: Locking in the clk API

> > For internal tree purposes, does .set_termios need to be atomic? Can it  
> > grab mutexes instead of spinlock?
> 
> I think I already answered that question above where I said "protect
> against the interrupt handler accessing the port->* stuff".

I'm not sure you answered it correctly however as the locking nowdays is
a bit different.

Architecturally the termios handling doesn't need a spin lock nor is it
called under one. In fact it's vital this is the case because of USB.

I see nothing in the 2.6.37 cpm_uart code that isn't trivially fixable.
There is already a mutex protecting termios serialization so all you seem
to need to do is call clk_set_rate after rather than before dropping the
lock surely ?

Oh if you are looking at the cpm_uart code and care about it the following
in the code isn't safe as tty could possibly go NULL and be freed under
you.

     struct tty_struct *tty = port->state->port.tty;

and you ought to be doing

	tty = tty_port_tty_get(&port->state->port);

	if (tty)
		blah;
	tty_kref_put(tty);	/* put NULL is a no-op anyway */

probably in the main irq handler and pass tty to the helpers that need it.

Alan
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