Subject: Re: [RESEND][RFC][PATCH] leds: fix oops race in led trigger
	registration
From: Richard Purdie <rpurdie@rpsys.net>
To: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Zdenek Kabelac <zdenek.kabelac@gmail.com>,
       Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
       linux-kernel@vger.kernel.org
In-Reply-To: <20080821071750.GA8046@localhost.localdomain>
References: <20080821071750.GA8046@localhost.localdomain>
Content-Type: text/plain
Date: Thu, 21 Aug 2008 09:06:54 +0100
Message-Id: <1219306014.5385.9.camel@dax.rpnet.com>
Mime-Version: 1.0
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Sender: linux-kernel-owner@vger.kernel.org
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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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