linux-kernel - Re: [PATCH v5 04/46] pwm: get rid of pwm->lock

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Date:	Tue, 12 Apr 2016 13:32:55 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	linux-pwm@...r.kernel.org,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>, linux-clk@...r.kernel.org,
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	linux-input@...r.kernel.org, Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
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	Alexander Shiyan <shc_work@...l.ru>, Milo Kim <milo.kim@...com>
Subject: Re: [PATCH v5 04/46] pwm: get rid of pwm->lock

Hi Thierry,

On Tue, 12 Apr 2016 13:22:46 +0200
Thierry Reding <thierry.reding@...il.com> wrote:

> On Wed, Mar 30, 2016 at 10:03:27PM +0200, Boris Brezillon wrote:
> > PWM devices are not protected against concurrent accesses. The lock in
> > pwm_device might let PWM users think it is, but it's actually only
> > protecting the enabled state.
> > 
> > Removing this lock should be fine as long as all PWM users are aware that
> > accesses to the PWM device have to be serialized, which seems to be the
> > case for all of them except the sysfs interface.
> > Patch the sysfs code by adding a lock to the pwm_export struct and making
> > sure it's taken for all accesses to the exported PWM device.
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
> > ---
> >  drivers/pwm/core.c  | 19 ++++--------------
> >  drivers/pwm/sysfs.c | 57 ++++++++++++++++++++++++++++++++++++++++++-----------
> >  include/linux/pwm.h |  2 --
> >  3 files changed, 50 insertions(+), 28 deletions(-)
> 
> This is a little overzealous. Only accesses that can cause races need to
> be protected by the lock. All of the *_show() callbacks don't modify the
> PWM device in any way, so there is no need to protect them against
> concurrent accesses.

This is probably true for this set of changes, but what will happen
when we'll switch to the atomic API? There's no guarantee that
pwm->state = *newstate is done atomically, and you may see a partially
updated state when calling pwm_get_state() while another thread is
calling pwm_apply_state().

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com