Re: [PATCH v6] ACPI: battery: prevent sysfs_add_battery re-entry on rapid events

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tuesday, September 23, 2025 7:12:03 PM CEST Rafael J. Wysocki wrote:
> On Tue, Sep 23, 2025 at 6:14 PM GuangFei Luo <luogf2025@163.com> wrote:
> >
> > The functions battery_hook_add_battery(), battery_hook_remove_battery(),
> > and sysfs_remove_battery() already acquire locks, so their internal
> > accesses are safe.
> 
> In fact, there are two locks in use, battery->sysfs_lock and
> hook_mutex.  The latter is used for managing hooks and the former is
> only used by sysfs_remove_battery(), so it only prevents that function
> from racing with another instance of itself.
> 
> I would suggest using battery->sysfs_lock for protecting battery->bat
> in general.
> 
> > acpi_battery_refresh() does check battery->bat, but its child
> > functions (sysfs_add_battery() and sysfs_remove_battery()) already
> > handle locking.
> 
> What locking?  Before the $subject patch, sysfs_add_battery() doesn't
> do any locking at all AFAICS.
> 
> > In acpi_battery_notify(), battery->bat has no lock. However, the
> > check of battery->bat is at the very end of the function. During
> > earlier calls, battery->bat has already been protected by locks, so
> > re-entry will not cause issues.
> 
> All of the battery->bat checks and the code depending on them need to
> go under the same lock.  I'd use battery->sysfs_lock for this as
> already mentioned above.

So my (untested) version of this fix is appended.

Note that it explicitly prevents acpi_battery_notify() from racing with
addition/removal, PM notifications, and resume.

---
 drivers/acpi/battery.c |   36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

--- a/drivers/acpi/battery.c
+++ b/drivers/acpi/battery.c
@@ -92,7 +92,7 @@ enum {
 
 struct acpi_battery {
 	struct mutex lock;
-	struct mutex sysfs_lock;
+	struct mutex update_lock;
 	struct power_supply *bat;
 	struct power_supply_desc bat_desc;
 	struct acpi_device *device;
@@ -904,15 +904,12 @@ static int sysfs_add_battery(struct acpi
 
 static void sysfs_remove_battery(struct acpi_battery *battery)
 {
-	mutex_lock(&battery->sysfs_lock);
-	if (!battery->bat) {
-		mutex_unlock(&battery->sysfs_lock);
+	if (!battery->bat)
 		return;
-	}
+
 	battery_hook_remove_battery(battery);
 	power_supply_unregister(battery->bat);
 	battery->bat = NULL;
-	mutex_unlock(&battery->sysfs_lock);
 }
 
 static void find_battery(const struct dmi_header *dm, void *private)
@@ -1072,6 +1069,9 @@ static void acpi_battery_notify(acpi_han
 
 	if (!battery)
 		return;
+
+	guard(mutex)(&battery->update_lock);
+
 	old = battery->bat;
 	/*
 	 * On Acer Aspire V5-573G notifications are sometimes triggered too
@@ -1094,21 +1094,22 @@ static void acpi_battery_notify(acpi_han
 }
 
 static int battery_notify(struct notifier_block *nb,
-			       unsigned long mode, void *_unused)
+			  unsigned long mode, void *_unused)
 {
 	struct acpi_battery *battery = container_of(nb, struct acpi_battery,
 						    pm_nb);
-	int result;
 
-	switch (mode) {
-	case PM_POST_HIBERNATION:
-	case PM_POST_SUSPEND:
+	if (mode == PM_POST_SUSPEND || mode == PM_POST_HIBERNATION) {
+		guard(mutex)(&battery->update_lock);
+
 		if (!acpi_battery_present(battery))
 			return 0;
 
 		if (battery->bat) {
 			acpi_battery_refresh(battery);
 		} else {
+			int result;
+
 			result = acpi_battery_get_info(battery);
 			if (result)
 				return result;
@@ -1120,7 +1121,6 @@ static int battery_notify(struct notifie
 
 		acpi_battery_init_alarm(battery);
 		acpi_battery_get_state(battery);
-		break;
 	}
 
 	return 0;
@@ -1198,6 +1198,8 @@ static int acpi_battery_update_retry(str
 {
 	int retry, ret;
 
+	guard(mutex)(&battery->update_lock);
+
 	for (retry = 5; retry; retry--) {
 		ret = acpi_battery_update(battery, false);
 		if (!ret)
@@ -1230,7 +1232,7 @@ static int acpi_battery_add(struct acpi_
 	if (result)
 		return result;
 
-	result = devm_mutex_init(&device->dev, &battery->sysfs_lock);
+	result = devm_mutex_init(&device->dev, &battery->update_lock);
 	if (result)
 		return result;
 
@@ -1262,6 +1264,8 @@ fail_pm:
 	device_init_wakeup(&device->dev, 0);
 	unregister_pm_notifier(&battery->pm_nb);
 fail:
+	guard(mutex)(&battery->update_lock);
+
 	sysfs_remove_battery(battery);
 
 	return result;
@@ -1281,6 +1285,9 @@ static void acpi_battery_remove(struct a
 
 	device_init_wakeup(&device->dev, 0);
 	unregister_pm_notifier(&battery->pm_nb);
+
+	guard(mutex)(&battery->update_lock);
+
 	sysfs_remove_battery(battery);
 }
 
@@ -1297,6 +1304,9 @@ static int acpi_battery_resume(struct de
 		return -EINVAL;
 
 	battery->update_time = 0;
+
+	guard(mutex)(&battery->update_lock);
+
 	acpi_battery_update(battery, true);
 	return 0;
 }