iio: core: fix a possible circular locking dependency
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:
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>