From 7231ed1a813e0a9d249bbbe58e66ca058aee83e1 Mon Sep 17 00:00:00 2001 From: Hans de Goede Date: Thu, 13 Aug 2015 18:53:37 +0200 Subject: [PATCH] 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: 93a291dfaf9c (ACPI / video: Move backlight notifier to video_detect.c) Signed-off-by: Hans de Goede Reported-and-tested-by: Sedat Dilek Signed-off-by: Rafael J. Wysocki --- drivers/acpi/video_detect.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c index 815f75ef2411..2922f1f252d5 100644 --- a/drivers/acpi/video_detect.c +++ b/drivers/acpi/video_detect.c @@ -32,6 +32,7 @@ #include #include #include +#include #include ACPI_MODULE_NAME("video"); @@ -41,6 +42,7 @@ void acpi_video_unregister_backlight(void); static bool backlight_notifier_registered; 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_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, 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 */ if (backlight->props.type == BACKLIGHT_RAW && - val == BACKLIGHT_REGISTERED && - acpi_video_get_backlight_type() != acpi_backlight_video) - acpi_video_unregister_backlight(); + val == BACKLIGHT_REGISTERED) + schedule_work(&backlight_notify_work); 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_UINT32_MAX, find_video, NULL, &video_caps, NULL); + INIT_WORK(&backlight_notify_work, + acpi_video_backlight_notify_work); backlight_nb.notifier_call = acpi_video_backlight_notify; backlight_nb.priority = 0; if (backlight_register_notifier(&backlight_nb) == 0) -- GitLab