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Subject[PATCH 5.5 049/150] hwmon: (acpi_power_meter) Fix lockdep splat
Date
From: Guenter Roeck <linux@roeck-us.net>

commit badcd4546d52ae4318f2bcfda0e47a1394b60e38 upstream.

Damien Le Moal reports a lockdep splat with the acpi_power_meter,
observed with Linux v5.5 and later.

======================================================
WARNING: possible circular locking dependency detected
5.6.0-rc2+ #629 Not tainted
------------------------------------------------------
python/1397 is trying to acquire lock:
ffff888619080070 (&resource->lock){+.+.}, at: show_power+0x3c/0xa0 [acpi_power_meter]

but task is already holding lock:
ffff88881643f188 (kn->count#119){++++}, at: kernfs_seq_start+0x6a/0x160

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (kn->count#119){++++}:
__kernfs_remove+0x626/0x7e0
kernfs_remove_by_name_ns+0x41/0x80
remove_attrs+0xcb/0x3c0 [acpi_power_meter]
acpi_power_meter_notify+0x1f7/0x310 [acpi_power_meter]
acpi_ev_notify_dispatch+0x198/0x1f3
acpi_os_execute_deferred+0x4d/0x70
process_one_work+0x7c8/0x1340
worker_thread+0x94/0xc70
kthread+0x2ed/0x3f0
ret_from_fork+0x24/0x30

-> #0 (&resource->lock){+.+.}:
__lock_acquire+0x20be/0x49b0
lock_acquire+0x127/0x340
__mutex_lock+0x15b/0x1350
show_power+0x3c/0xa0 [acpi_power_meter]
dev_attr_show+0x3f/0x80
sysfs_kf_seq_show+0x216/0x410
seq_read+0x407/0xf90
vfs_read+0x152/0x2c0
ksys_read+0xf3/0x1d0
do_syscall_64+0x95/0x1010
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(kn->count#119);
lock(&resource->lock);
lock(kn->count#119);
lock(&resource->lock);

*** DEADLOCK ***
4 locks held by python/1397:
#0: ffff8890242d64e0 (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x9b/0xb0
#1: ffff889040be74e0 (&p->lock){+.+.}, at: seq_read+0x6b/0xf90
#2: ffff8890448eb880 (&of->mutex){+.+.}, at: kernfs_seq_start+0x47/0x160
#3: ffff88881643f188 (kn->count#119){++++}, at: kernfs_seq_start+0x6a/0x160

stack backtrace:
CPU: 10 PID: 1397 Comm: python Not tainted 5.6.0-rc2+ #629
Hardware name: Supermicro Super Server/X11DPL-i, BIOS 3.1 05/21/2019
Call Trace:
dump_stack+0x97/0xe0
check_noncircular+0x32e/0x3e0
? print_circular_bug.isra.0+0x1e0/0x1e0
? unwind_next_frame+0xb9a/0x1890
? entry_SYSCALL_64_after_hwframe+0x49/0xbe
? graph_lock+0x79/0x170
? __lockdep_reset_lock+0x3c0/0x3c0
? mark_lock+0xbc/0x1150
__lock_acquire+0x20be/0x49b0
? mark_held_locks+0xe0/0xe0
? stack_trace_save+0x91/0xc0
lock_acquire+0x127/0x340
? show_power+0x3c/0xa0 [acpi_power_meter]
? device_remove_bin_file+0x10/0x10
? device_remove_bin_file+0x10/0x10
__mutex_lock+0x15b/0x1350
? show_power+0x3c/0xa0 [acpi_power_meter]
? show_power+0x3c/0xa0 [acpi_power_meter]
? mutex_lock_io_nested+0x11f0/0x11f0
? lock_downgrade+0x6a0/0x6a0
? kernfs_seq_start+0x47/0x160
? lock_acquire+0x127/0x340
? kernfs_seq_start+0x6a/0x160
? device_remove_bin_file+0x10/0x10
? show_power+0x3c/0xa0 [acpi_power_meter]
show_power+0x3c/0xa0 [acpi_power_meter]
dev_attr_show+0x3f/0x80
? memset+0x20/0x40
sysfs_kf_seq_show+0x216/0x410
seq_read+0x407/0xf90
? security_file_permission+0x16f/0x2c0
vfs_read+0x152/0x2c0

Problem is that reading an attribute takes the kernfs lock in the kernfs
code, then resource->lock in the driver. During an ACPI notification, the
opposite happens: The resource lock is taken first, followed by the kernfs
lock when sysfs attributes are removed and re-created. Presumably this is
now seen due to some locking related changes in kernfs after v5.4, but it
was likely always a problem.

Fix the problem by not blindly acquiring the lock in the notification
function. It is only needed to protect the various update functions.
However, those update functions are called anyway when sysfs attributes
are read. This means that we can just stop calling those functions from
the notifier, and the resource lock in the notifier function is no longer
needed.

That leaves two situations:

First, METER_NOTIFY_CONFIG removes and re-allocates capability strings.
While it did so under the resource lock, _displaying_ those strings was not
protected, creating a race condition. To solve this problem, selectively
protect both removal/creation and reporting of capability attributes with
the resource lock.

Second, removing and re-creating the attribute files is no longer protected
by the resource lock. That doesn't matter since access to each individual
attribute is protected by the kernfs lock. Userspace may get messed up if
attributes disappear and reappear under its nose, but that is not different
than today, and there is nothing we can do about it without major driver
restructuring.

Last but not least, when removing the driver, remove attribute functions
first, then release capability strings. This avoids yet another race
condition.

Reported-by: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: stable@vger.kernel.org # v5.5+
Tested-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/hwmon/acpi_power_meter.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

--- a/drivers/hwmon/acpi_power_meter.c
+++ b/drivers/hwmon/acpi_power_meter.c
@@ -355,7 +355,9 @@ static ssize_t show_str(struct device *d
struct acpi_device *acpi_dev = to_acpi_device(dev);
struct acpi_power_meter_resource *resource = acpi_dev->driver_data;
acpi_string val;
+ int ret;

+ mutex_lock(&resource->lock);
switch (attr->index) {
case 0:
val = resource->model_number;
@@ -372,8 +374,9 @@ static ssize_t show_str(struct device *d
val = "";
break;
}
-
- return sprintf(buf, "%s\n", val);
+ ret = sprintf(buf, "%s\n", val);
+ mutex_unlock(&resource->lock);
+ return ret;
}

static ssize_t show_val(struct device *dev,
@@ -817,11 +820,12 @@ static void acpi_power_meter_notify(stru

resource = acpi_driver_data(device);

- mutex_lock(&resource->lock);
switch (event) {
case METER_NOTIFY_CONFIG:
+ mutex_lock(&resource->lock);
free_capabilities(resource);
res = read_capabilities(resource);
+ mutex_unlock(&resource->lock);
if (res)
break;

@@ -830,15 +834,12 @@ static void acpi_power_meter_notify(stru
break;
case METER_NOTIFY_TRIP:
sysfs_notify(&device->dev.kobj, NULL, POWER_AVERAGE_NAME);
- update_meter(resource);
break;
case METER_NOTIFY_CAP:
sysfs_notify(&device->dev.kobj, NULL, POWER_CAP_NAME);
- update_cap(resource);
break;
case METER_NOTIFY_INTERVAL:
sysfs_notify(&device->dev.kobj, NULL, POWER_AVG_INTERVAL_NAME);
- update_avg_interval(resource);
break;
case METER_NOTIFY_CAPPING:
sysfs_notify(&device->dev.kobj, NULL, POWER_ALARM_NAME);
@@ -848,7 +849,6 @@ static void acpi_power_meter_notify(stru
WARN(1, "Unexpected event %d\n", event);
break;
}
- mutex_unlock(&resource->lock);

acpi_bus_generate_netlink_event(ACPI_POWER_METER_CLASS,
dev_name(&device->dev), event, 0);
@@ -912,8 +912,8 @@ static int acpi_power_meter_remove(struc
resource = acpi_driver_data(device);
hwmon_device_unregister(resource->hwmon_dev);

- free_capabilities(resource);
remove_attrs(resource);
+ free_capabilities(resource);

kfree(resource);
return 0;

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