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Commit 7231ed1a authored by Hans de Goede's avatar Hans de Goede Committed by Rafael J. Wysocki
Browse files

ACPI / video: Fix circular lock dependency issue in the video-detect code 



Before this commit, the following would happen:

 a) acpi_video_get_backlight_type() gets called
 b) acpi_video_get_backlight_type() calls acpi_video_init_backlight_type()
 c) acpi_video_init_backlight_type() locks its function static init_mutex
 d) acpi_video_init_backlight_type() calls backlight_register_notifier()
 e) backlight_register_notifier() takes its notifier-chain lock

And when the backlight notifier chain gets called we've:

 1) blocking_notifier_call_chain() gets called
 2) blocking_notifier_call_chain() takes the notifier-chain lock
 3) blocking_notifier_call_chain() calls acpi_video_backlight_notify()
 4) acpi_video_backlight_notify() calls acpi_video_get_backlight_type()
 5) acpi_video_get_backlight_type() calls acpi_video_init_backlight_type()
 6) acpi_video_init_backlight_type() locks its function static init_mutex

So in the first call sequence we have:

 a) init_mutex gets locked
 b) notifier-chain gets locked

and in the second call sequence we have:

 1) notifier-chain gets locked
 2) init_mutex gets locked

And we've a circular locking dependency. This specific locking dependency
is fixable without using the big hammer otherwise known as a workqueue,
but further analysis shows a similar problem with the backlight notifier
chain lock vs register_count_mutex from drivers/acpi/acpi_video.c,
and fixing that becomes problematic.

So this commit simply fixes this with the big hammer, performance
wise this is a non issue as we expect the work to get scheduled
exactly zero or one times during normal system use.

Fixes: 93a291df (ACPI / video: Move backlight notifier to video_detect.c)
Signed-off-by: default avatarHans de Goede <hdegoede@redhat.com>
Reported-and-tested-by: default avatarSedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
parent f7644cbf

drivers/acpi/video_detect.c

+13 −3
Original line number Original line Diff line number Diff line
@@ -32,6 +32,7 @@ 
#include <linux/module.h>

#include <linux/module.h>

#include <linux/pci.h>

#include <linux/pci.h>

#include <linux/types.h>

#include <linux/types.h>

#include <linux/workqueue.h>

#include <acpi/video.h>

#include <acpi/video.h>





ACPI_MODULE_NAME("video");

ACPI_MODULE_NAME("video");
@@ -41,6 +42,7 @@ void acpi_video_unregister_backlight(void); 




static bool backlight_notifier_registered;

static bool backlight_notifier_registered;

static struct notifier_block backlight_nb;

static struct notifier_block backlight_nb;

static struct work_struct backlight_notify_work;





static enum acpi_backlight_type acpi_backlight_cmdline = acpi_backlight_undef;

static enum acpi_backlight_type acpi_backlight_cmdline = acpi_backlight_undef;

static enum acpi_backlight_type acpi_backlight_dmi = acpi_backlight_undef;

static enum acpi_backlight_type acpi_backlight_dmi = acpi_backlight_undef;
@@ -262,6 +264,13 @@ static const struct dmi_system_id video_detect_dmi_table[] = { 
	{ },

	{ },

};

};





/* This uses a workqueue to avoid various locking ordering issues */

static void acpi_video_backlight_notify_work(struct work_struct *work)

{

	if (acpi_video_get_backlight_type() != acpi_backlight_video)

		acpi_video_unregister_backlight();

}



static int acpi_video_backlight_notify(struct notifier_block *nb,

static int acpi_video_backlight_notify(struct notifier_block *nb,

				       unsigned long val, void *bd)

				       unsigned long val, void *bd)

{

{
@@ -269,9 +278,8 @@ static int acpi_video_backlight_notify(struct notifier_block *nb, 




	/* A raw bl registering may change video -> native */

	/* A raw bl registering may change video -> native */

	if (backlight->props.type == BACKLIGHT_RAW &&

	if (backlight->props.type == BACKLIGHT_RAW &&

	    val == BACKLIGHT_REGISTERED &&

	    val == BACKLIGHT_REGISTERED)

	    acpi_video_get_backlight_type() != acpi_backlight_video)

		schedule_work(&backlight_notify_work);

		acpi_video_unregister_backlight();





	return NOTIFY_OK;

	return NOTIFY_OK;

}

}
@@ -304,6 +312,8 @@ enum acpi_backlight_type acpi_video_get_backlight_type(void) 
		acpi_walk_namespace(ACPI_TYPE_DEVICE, ACPI_ROOT_OBJECT,

		acpi_walk_namespace(ACPI_TYPE_DEVICE, ACPI_ROOT_OBJECT,

				    ACPI_UINT32_MAX, find_video, NULL,

				    ACPI_UINT32_MAX, find_video, NULL,

				    &video_caps, NULL);

				    &video_caps, NULL);

		INIT_WORK(&backlight_notify_work,

			  acpi_video_backlight_notify_work);

		backlight_nb.notifier_call = acpi_video_backlight_notify;

		backlight_nb.notifier_call = acpi_video_backlight_notify;

		backlight_nb.priority = 0;

		backlight_nb.priority = 0;

		if (backlight_register_notifier(&backlight_nb) == 0)

		if (backlight_register_notifier(&backlight_nb) == 0)