ACPI: button: Fix lid_device value leak past driver removal

Static variable lid_device is set when the ACPI button driver probes
the last lid device (under the assumptions that there will be only
one lid device in the system) and never cleared, but in principle it
should be reset when the driver unbinds from the lid device pointed
to by it.

Address that and add locking that is needed to clear and set that
variable safely.

Fixes: 7e12715 ("ACPI button: provide lid status functions")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/6281379.lOV4Wx5bFT@rafael.j.wysocki

Author: rafaeljw

drivers/acpi/button.c

@@ -182,7 +182,6 @@ struct acpi_button {
 	bool gpe_enabled;
 };
 
-static struct acpi_device *lid_device;
 static long lid_init_state = -1;
 
 static unsigned long lid_report_interval __read_mostly = 500;
@@ -378,9 +377,29 @@ static int acpi_button_remove_fs(struct acpi_button *button)
 	return 0;
 }
 
+static struct acpi_device *lid_device;
+static DEFINE_MUTEX(acpi_lid_lock);
+
+static void acpi_lid_save(struct acpi_device *adev)
+{
+	guard(mutex)(&acpi_lid_lock);
+
+	lid_device = adev;
+}
+
+static void acpi_lid_forget(struct acpi_device *adev)
+{
+	guard(mutex)(&acpi_lid_lock);
+
+	if (lid_device == adev)
+		lid_device = NULL;
+}
+
 /* Driver Interface */
 int acpi_lid_open(void)
 {
+	guard(mutex)(&acpi_lid_lock);
+
 	if (!lid_device)
 		return -ENODEV;
 
@@ -674,7 +693,7 @@ static int acpi_button_probe(struct platform_device *pdev)
 		 * This assumes there's only one lid device, or if there are
 		 * more we only care about the last one...
 		 */
-		lid_device = device;
+		acpi_lid_save(device);
 	}
 
 	pr_info("%s [%s]\n", name, acpi_device_bid(device));
@@ -696,6 +715,9 @@ static void acpi_button_remove(struct platform_device *pdev)
 	struct acpi_button *button = platform_get_drvdata(pdev);
 	struct acpi_device *adev = button->adev;
 
+	if (button->type == ACPI_BUTTON_TYPE_LID)
+		acpi_lid_forget(adev);
+
 	switch (adev->device_type) {
 	case ACPI_BUS_TYPE_POWER_BUTTON:
 		acpi_remove_fixed_event_handler(ACPI_EVENT_POWER_BUTTON,