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: ac917a81 ("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>