linux-kernel - Re: [RESEND][RFC][PATCH] leds: fix oops race in led trigger registration

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Date:	Tue, 26 Aug 2008 14:36:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Richard Purdie <rpurdie@...ys.net>
Cc:	vegard.nossum@...il.com, zdenek.kabelac@...il.com, hmh@....eng.br,
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND][RFC][PATCH] leds: fix oops race in led trigger
 registration

On Thu, 21 Aug 2008 09:06:54 +0100
Richard Purdie <rpurdie@...ys.net> wrote:

> Hi,
> 
> On Thu, 2008-08-21 at 09:17 +0200, Vegard Nossum wrote:
> > I resubmit this, which was posted a month ago, with no responses from
> > the maintainer (and no recent fixes in this area either). I have not
> > tested it myself, but I am reasonably confident that this is an
> > obviously correct fix. I guess it would not hurt to exercise it as well.
> > 
> > Zdenek: Have recent kernels also been showing this behaviour? Did the
> > patch fix the problem for you? You said something about a new issue
> > showing up, do you think that was related to this patch?
> > 
> > Also, just one question (since I'm newbie at this): Is it dangerous to
> > register the &dev_attr_trigger device file before we put the device on
> > the list or initialize it completely? If so, the initialization should
> > probably be moved all to the top before the file is made public.
> 
> This patch isn't quite right since whilst it fixes one race, it opens up
> a number of other potential problems. We definitely should have the led
> listed in leds_list before calling the various functions and the
> existing init order is correct in that sense.
> 
> I suspect your problem is caused by something calling a trigger
> registration between the point its added to the list and the
> init_rwsem(). The best solution is to move the init_rwsem() to earlier
> in the function. Yes, that means some ifdef ugliness but so be it. Could
> you see whether you still see the problem with the patch below?
> 
> Cheers,
> 
> Richard
> 
> 
> diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> index 559a408..c9d8219 100644
> --- a/drivers/leds/led-class.c
> +++ b/drivers/leds/led-class.c
> @@ -113,6 +113,9 @@ int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)
>  	if (rc)
>  		goto err_out;
>  
> +#ifdef CONFIG_LEDS_TRIGGERS
> +	init_rwsem(&led_cdev->trigger_lock);
> +#endif
>  	/* add to the list of leds */
>  	down_write(&leds_list_lock);
>  	list_add_tail(&led_cdev->node, &leds_list);
> @@ -121,8 +124,6 @@ int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)
>  	led_update_brightness(led_cdev);
>  
>  #ifdef CONFIG_LEDS_TRIGGERS
> -	init_rwsem(&led_cdev->trigger_lock);
> -
>  	rc = device_create_file(led_cdev->dev, &dev_attr_trigger);
>  	if (rc)
>  		goto err_out_led_list;
> 

??

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