linux-kernel - Re: use of preempt_count instead of in

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Message-Id: <200803181206.14162.david-b@pacbell.net>
Date:	Tue, 18 Mar 2008 11:06:13 -0800
From:	David Brownell <david-b@...bell.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Richard Purdie <rpurdie@...ys.net>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

On Tuesday 18 March 2008, Andrew Morton wrote:
> On Sun, 16 Mar 2008 11:46:23 -0800 David Brownell <david-b@...bell.net> wrote:
> 
> > On Sunday 16 March 2008, Henrique de Moraes Holschuh wrote:
> > > Is the use of "if (preempt_count())" to know when to defer led gpio work to
> > > a workqueue needed? __Shouldn't "if (in_atomic())" be enough?
> > 
> > At this point, I don't know of any such reason.
> > 
> > I remember hunting for the right heuristic, and settling on
> > that one for reasons that I can't recall now.  They may even
> > be no longer applicable.
> 
> Both are incorrect. 

So something like the appended patch would seem "better"?


> <greps for in_atomic>
> 
> omigawd, what have we done, and how can we fix it? :(

 
==============
It appears that we can't just check to see if we're in a task
context ... so instead of trying that, just make the relevant
leds always schedule a little worklet.

Signed-off-by: David Brownell <dbrownell@...rs.sourceforge.net>
---
 drivers/leds/leds-gpio.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- g26.orig/drivers/leds/leds-gpio.c	2008-03-18 01:32:08.000000000 -0700
+++ g26/drivers/leds/leds-gpio.c	2008-03-18 02:01:23.000000000 -0700
@@ -49,13 +49,13 @@ static void gpio_led_set(struct led_clas
 	if (led_dat->active_low)
 		level = !level;
 
-	/* setting GPIOs with I2C/etc requires a preemptible task context */
+	/* Setting GPIOs with I2C/etc requires a task context, and we don't
+	 * seem to have a reliable way to know if we're already in one; so
+	 * let's just assume the worst.
+	 */
 	if (led_dat->can_sleep) {
-		if (preempt_count()) {
-			led_dat->new_level = level;
-			schedule_work(&led_dat->work);
-		} else
-			gpio_set_value_cansleep(led_dat->gpio, level);
+		led_dat->new_level = level;
+		schedule_work(&led_dat->work);
 	} else
 		gpio_set_value(led_dat->gpio, level);
 }


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