linux-kernel - Re: [PATCH] MXS: Convert mutexes in clock.c to spinlocks

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Date:	Mon, 19 Dec 2011 08:22:25 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Marek Vasut <marek.vasut@...il.com>
Cc:	Shawn Guo <shawn.guo@...escale.com>, Wolfgang Denk <wd@...x.de>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, Huang Shijie <b32955@...escale.com>,
	linux-arm-kernel@...ts.infradead.org,
	Shawn Guo <shawn.guo@...aro.org>,
	Stefano Babic <sbabic@...x.de>
Subject: Re: [PATCH] MXS: Convert mutexes in clock.c to spinlocks

On Mon, Dec 19, 2011 at 05:03:45AM +0100, Marek Vasut wrote:
> > On Sun, Dec 18, 2011 at 03:06:13PM +0100, Marek Vasut wrote:
> > > The mutexes can't be safely used under certain circumstances. I noticed
> > > this
> > 
> > > issue during some network instability at home:
> > Yes, this is a known issue.  And there was some discussion[1] about
> > why mutex is needed.
> 
> Thanks for pointing this out, I was unaware of it.
> 
> > But I really have not thought about why we can
> > not use spinlock only, since using mutex only leads to the issue we
> > are seeing here, and using spinlock in enable/disable and mutex in
> > rate/parent will not work, because the mxs clocks have enable/disable
> > and rate/parent functions access the same register.  I know it's not
> > good to hold spinlock in rate/parent functions for a long time, but
> > do we have a way around rather than using spinlock for both sets of
> > functions?
> 
> Yea, spinlock is not good either. On the other hand, is it really held for so 
> long ?

There is another solution to this, which I've pointed out before when
this has come up:

1. Convert all your drivers to _also_ use clk_prepare()/clk_unprepare().
   You need to do this anyway as it will become mandatory for the common
   clk stuff.

2. Rename your existing clk_enable()/clk_disable() implementation to
   clk_prepare()/clk_unprepare().  Ensure CONFIG_HAVE_CLK_PREPARE is
   selected.

3. Provide a new no-op clk_enable()/clk_disable() functions.

This fixes the issue because clk_prepare()/clk_unprepare() must only be
called from process contexts, whereas clk_enable()/clk_disable() may be
called from atomic contexts as well.
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