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SubjectRe: [RESEND][RFC][PATCH] leds: fix oops race in led trigger registration

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?



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;

+ init_rwsem(&led_cdev->trigger_lock);
/* add to the list of leds */
list_add_tail(&led_cdev->node, &leds_list);
@@ -121,8 +124,6 @@ int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)

- 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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