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:Hans de Goede <hdegoede@redhat.com> Reported-and-tested-by:
Sedat Dilek <sedat.dilek@gmail.com> Signed-off-by:
Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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