iio: core: add separate lockdep class for info_exist_lock

When one iio device is a consumer of another, it is possible that
the ->info_exist_lock of both ends up being taken when reading the
value of the consumer device.

Since they currently belong to the same lockdep class (being
initialized in a single location with mutex_init()), that results in a
lockdep warning

CPU0
----
lock(&iio_dev_opaque->info_exist_lock);
lock(&iio_dev_opaque->info_exist_lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

4 locks held by sensors/414:
#0: c31fd6dc (&p->lock){+.+.}-{3:3}, at: seq_read_iter+0x44/0x4e4
#1: c4f5a1c4 (&of->mutex){+.+.}-{3:3}, at: kernfs_seq_start+0x1c/0xac
#2: c2827548 (kn->active#34){.+.+}-{0:0}, at: kernfs_seq_start+0x30/0xac
#3: c1dd2b68 (&iio_dev_opaque->info_exist_lock){+.+.}-{3:3}, at: iio_read_channel_processed_scale+0x24/0xd8

stack backtrace:
CPU: 0 UID: 0 PID: 414 Comm: sensors Not tainted 6.17.11 #5 NONE
Hardware name: Generic AM33XX (Flattened Device Tree)
Call trace:
unwind_backtrace from show_stack+0x10/0x14
show_stack from dump_stack_lvl+0x44/0x60
dump_stack_lvl from print_deadlock_bug+0x2b8/0x334
print_deadlock_bug from __lock_acquire+0x13a4/0x2ab0
__lock_acquire from lock_acquire+0xd0/0x2c0
lock_acquire from __mutex_lock+0xa0/0xe8c
__mutex_lock from mutex_lock_nested+0x1c/0x24
mutex_lock_nested from iio_read_channel_raw+0x20/0x6c
iio_read_channel_raw from rescale_read_raw+0x128/0x1c4
rescale_read_raw from iio_channel_read+0xe4/0xf4
iio_channel_read from iio_read_channel_processed_scale+0x6c/0xd8
iio_read_channel_processed_scale from iio_hwmon_read_val+0x68/0xbc
iio_hwmon_read_val from dev_attr_show+0x18/0x48
dev_attr_show from sysfs_kf_seq_show+0x80/0x110
sysfs_kf_seq_show from seq_read_iter+0xdc/0x4e4
seq_read_iter from vfs_read+0x238/0x2e4
vfs_read from ksys_read+0x6c/0xec
ksys_read from ret_fast_syscall+0x0/0x1c

Just as the mlock_key already has its own lockdep class, add a
lock_class_key for the info_exist mutex.

Note that this has in theory been a problem since before IIO first
left staging, but it only occurs when a chain of consumers is in use
and that is not often done.

Fixes: ac917a81117c ("staging:iio:core set the iio_dev.info pointer to null on unregister under lock.")
Signed-off-by: Rasmus Villemoes <[e-mail]>
Reviewed-by: Peter Rosin <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-core.c

···
 	mutex_destroy(&iio_dev_opaque->info_exist_lock);
 	mutex_destroy(&iio_dev_opaque->mlock);
 
+	lockdep_unregister_key(&iio_dev_opaque->info_exist_key);
 	lockdep_unregister_key(&iio_dev_opaque->mlock_key);
 
 	ida_free(&iio_ida, iio_dev_opaque->id);
···
 	INIT_LIST_HEAD(&iio_dev_opaque->ioctl_handlers);
 
 	lockdep_register_key(&iio_dev_opaque->mlock_key);
+	lockdep_register_key(&iio_dev_opaque->info_exist_key);
 
 	mutex_init_with_key(&iio_dev_opaque->mlock, &iio_dev_opaque->mlock_key);
-	mutex_init(&iio_dev_opaque->info_exist_lock);
+	mutex_init_with_key(&iio_dev_opaque->info_exist_lock, &iio_dev_opaque->info_exist_key);
 
 	indio_dev->dev.parent = parent;
 	indio_dev->dev.type = &iio_device_type;

include/linux/iio/iio-opaque.h

···
  * @mlock:			lock used to prevent simultaneous device state changes
  * @mlock_key:			lockdep class for iio_dev lock
  * @info_exist_lock:		lock to prevent use during removal
+ * @info_exist_key:		lockdep class for info_exist lock
  * @trig_readonly:		mark the current trigger immutable
  * @event_interface:		event chrdevs associated with interrupt lines
  * @attached_buffers:		array of buffers statically attached by the driver
···
 	struct mutex			mlock;
 	struct lock_class_key		mlock_key;
 	struct mutex			info_exist_lock;
+	struct lock_class_key		info_exist_key;
 	bool				trig_readonly;
 	struct iio_event_interface	*event_interface;
 	struct iio_buffer		**attached_buffers;