[PATCH 4.14 25/70] iio: core: fix a possible circular locking dependency

Greg Kroah-Hartman
2019-04-24T17:28:10Z

From: Fabrice Gasnier <fabrice.gasnier@st.com>

commit 7f75591fc5a123929a29636834d1bcb8b5c9fee3 upstream.

This fixes a possible circular locking dependency detected warning seen
with:
- CONFIG_PROVE_LOCKING=y
- consumer/provider IIO devices (ex: "voltage-divider" consumer of "adc")

When using the IIO consumer interface, e.g. iio_channel_get(), the consumer
device will likely call iio_read_channel_raw() or similar that rely on
'info_exist_lock' mutex.

typically:
...
mutex_lock(&chan->indio_dev->info_exist_lock);
if (chan->indio_dev->info == NULL) {
ret = -ENODEV;
goto err_unlock;
}
ret = do_some_ops()
err_unlock:
mutex_unlock(&chan->indio_dev->info_exist_lock);
return ret;
...

Same mutex is also hold in iio_device_unregister().

The following deadlock warning happens when:
- the consumer device has called an API like iio_read_channel_raw()
at least once.
- the consumer driver is unregistered, removed (unbind from sysfs)

======================================================
WARNING: possible circular locking dependency detected
4.19.24 #577 Not tainted
------------------------------------------------------
sh/372 is trying to acquire lock:
(kn->count#30){++++}, at: kernfs_remove_by_name_ns+0x3c/0x84

but task is already holding lock:
(&dev->info_exist_lock){+.+.}, at: iio_device_unregister+0x18/0x60

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&dev->info_exist_lock){+.+.}:
__mutex_lock+0x70/0xa3c
mutex_lock_nested+0x1c/0x24
iio_read_channel_raw+0x1c/0x60
iio_read_channel_info+0xa8/0xb0
dev_attr_show+0x1c/0x48
sysfs_kf_seq_show+0x84/0xec
seq_read+0x154/0x528
__vfs_read+0x2c/0x15c
vfs_read+0x8c/0x110
ksys_read+0x4c/0xac
ret_fast_syscall+0x0/0x28
0xbedefb60

-> #0 (kn->count#30){++++}:
lock_acquire+0xd8/0x268
__kernfs_remove+0x288/0x374
kernfs_remove_by_name_ns+0x3c/0x84
remove_files+0x34/0x78
sysfs_remove_group+0x40/0x9c
sysfs_remove_groups+0x24/0x34
device_remove_attrs+0x38/0x64
device_del+0x11c/0x360
cdev_device_del+0x14/0x2c
iio_device_unregister+0x24/0x60
release_nodes+0x1bc/0x200
device_release_driver_internal+0x1a0/0x230
unbind_store+0x80/0x130
kernfs_fop_write+0x100/0x1e4
__vfs_write+0x2c/0x160
vfs_write+0xa4/0x17c
ksys_write+0x4c/0xac
ret_fast_syscall+0x0/0x28
0xbe906840

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&dev->info_exist_lock);
lock(kn->count#30);
lock(&dev->info_exist_lock);
lock(kn->count#30);

*** DEADLOCK ***
...

cdev_device_del() can be called without holding the lock. It should be safe
as info_exist_lock prevents kernelspace consumers to use the exported
routines during/after provider removal. cdev_device_del() is for userspace.

Help to reproduce:
See example: Documentation/devicetree/bindings/iio/afe/voltage-divider.txt
sysv {
compatible = "voltage-divider";
io-channels = <&adc 0>;
output-ohms = <22>;
full-ohms = <222>;
};

First, go to iio:deviceX for the "voltage-divider", do one read:
$ cd /sys/bus/iio/devices/iio:deviceX
$ cat in_voltage0_raw

Then, unbind the consumer driver. It triggers above deadlock warning.
$ cd /sys/bus/platform/drivers/iio-rescale/
$ echo sysv > unbind

Note I don't actually expect stable will pick this up all the
way back into IIO being in staging, but if's probably valid that
far back.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Fixes: ac917a81117c ("staging:iio:core set the iio_dev.info pointer to null on unregister")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/iio/industrialio-core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1741,10 +1741,10 @@ EXPORT_SYMBOL(iio_device_register);
**/
void iio_device_unregister(struct iio_dev *indio_dev)
{
- mutex_lock(&indio_dev->info_exist_lock);
-
cdev_device_del(&indio_dev->chrdev, &indio_dev->dev);

+ mutex_lock(&indio_dev->info_exist_lock);
+
iio_device_unregister_debugfs(indio_dev);

iio_disable_all_buffers(indio_dev);