iio: core: add a class hierarchy on iio device lock

The aim of this patch is to correct a recursive locking warning,
detected when setting CONFIG_PROVE_LOCKING flag (as shown in message below).
This message was initially triggered by the following call sequence
in stm32-dfsdm-adc.c driver, when using IIO hardware consumer interface.

in stm32_dfsdm_read_raw()
	iio_device_claim_direct_mode
		mutex_lock(&indio_dev->mlock);			-> lock on dfsdm device
	iio_hw_consumer_enable
		iio_update_buffers
			mutex_lock(&indio_dev->mlock);		-> lock on hw consumer device

Here two instances of the same lock class are requested
on two different objects.
The locking validator needs to be informed of the nesting level
of each lock to avoid a false positive.

This patch introduces a class hierarchy in iio device lock,
assuming that hardware consumer is at a lower level than iio device.

[   52.086174]
[   52.086223] ============================================
[   52.091516] WARNING: possible recursive locking detected
[   52.096825] 4.19.49 torvalds#162 Not tainted
[   52.100384] --------------------------------------------
[   52.105691] cat/823 is trying to acquire lock:
[   52.110132] 37acb703 (&dev->mlock){+.+.}, at: iio_update_buffers+0x3c/0xd0
[   52.116995]
[   52.116995] but task is already holding lock:
[   52.122821] 368bb908 (&dev->mlock){+.+.}, at: iio_device_claim_direct_mode+0x18/0x34
[   52.130560]
[   52.130560] other info that might help us debug this:
[   52.137083]  Possible unsafe locking scenario:
[   52.137083]
[   52.142995]        CPU0
[   52.145430]        ----
[   52.147864]   lock(&dev->mlock);
[   52.151082]   lock(&dev->mlock);
[   52.154301]
[   52.154301]  * DEADLOCK *
[   52.154301]
[   52.160215]  May be due to missing lock nesting notation
[   52.160215]
[   52.167000] 5 locks held by cat/823:
[   52.170563]  #0: 96d6554b (&p->lock){+.+.}, at: seq_read+0x34/0x51c
[   52.176824]  #1: 3cf6739a (&of->mutex){+.+.}, at: kernfs_seq_start+0x1c/0x8c
[   52.183866]  #2: a6090e0a (kn->count#29){.+.+}, at: kernfs_seq_start+0x24/0x8c
[   52.191083]  #3: 368bb908 (&dev->mlock){+.+.}, at: iio_device_claim_direct_mode+0x18/0x34
[   52.199257]  #4: 77e2bcfe (&dev->info_exist_lock){+.+.}, at: iio_update_buffers+0x30/0xd0
[   52.207431]
[   52.207431] stack backtrace:
[   52.211787] CPU: 0 PID: 823 Comm: cat Not tainted 4.19.49 torvalds#162
[   52.217606] Hardware name: STM32 (Device Tree Support)
[   52.222756] [<c0112420>] (unwind_backtrace) from [<c010df5c>] (show_stack+0x10/0x14)
[   52.230487] [<c010df5c>] (show_stack) from [<c0af5c88>] (dump_stack+0xc4/0xf0)
[   52.237703] [<c0af5c88>] (dump_stack) from [<c01865bc>] (__lock_acquire+0x874/0x1344)
[   52.245525] [<c01865bc>] (__lock_acquire) from [<c0187be8>] (lock_acquire+0xd8/0x268)
[   52.253353] [<c0187be8>] (lock_acquire) from [<c0b0dcf8>] (__mutex_lock+0x70/0xab0)
[   52.261005] [<c0b0dcf8>] (__mutex_lock) from [<c0b0e754>] (mutex_lock_nested+0x1c/0x24)
[   52.269001] [<c0b0e754>] (mutex_lock_nested) from [<c09282b8>] (iio_update_buffers+0x3c/0xd0)
[   52.277523] [<c09282b8>] (iio_update_buffers) from [<c09329cc>] (iio_hw_consumer_enable+0x34/0x70)
[   52.286476] [<c09329cc>] (iio_hw_consumer_enable) from [<c0932134>] (stm32_dfsdm_read_raw+0xf4/0x3fc)
[   52.295695] [<c0932134>] (stm32_dfsdm_read_raw) from [<c0922eb4>] (iio_read_channel_info+0xa8/0xb0)
[   52.304738] [<c0922eb4>] (iio_read_channel_info) from [<c067a7fc>] (dev_attr_show+0x1c/0x48)
[   52.313170] [<c067a7fc>] (dev_attr_show) from [<c03724a4>] (sysfs_kf_seq_show+0x84/0xec)
[   52.321256] [<c03724a4>] (sysfs_kf_seq_show) from [<c0312afc>] (seq_read+0x154/0x51c)
[   52.329082] [<c0312afc>] (seq_read) from [<c02e7a00>] (__vfs_read+0x2c/0x15c)
[   52.336209] [<c02e7a00>] (__vfs_read) from [<c02e7bc0>] (vfs_read+0x90/0x15c)
[   52.343339] [<c02e7bc0>] (vfs_read) from [<c02e81ac>] (ksys_read+0x5c/0xdc)
[   52.350296] [<c02e81ac>] (ksys_read) from [<c0101000>] (ret_fast_syscall+0x0/0x28)
[   52.357852] Exception stack(0xe5761fa8 to 0xe5761ff0)
[   52.362904] 1fa0:                   0000006c 7ff00000 00000003 b6e06000 00020000 00000000
[   52.371077] 1fc0: 0000006c 7ff00000 00020000 00000003 00000003 00000000 00020000 00000000
[   52.379245] 1fe0: 00000003 beb6e790 b6eb17b7 b6e3e6c6

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>

drivers/iio/buffer/industrialio-hw-consumer.c

@@ -101,6 +101,7 @@ struct iio_hw_consumer *iio_hw_consumer_alloc(struct device *dev)
 
 	chan = &hwc->channels[0];
 	while (chan->indio_dev) {
+		chan->indio_dev->mutex_class = IIO_MUTEX_HWC;
 		buf = iio_hw_consumer_get_buffer(hwc, chan->indio_dev);
 		if (!buf) {
 			ret = -ENOMEM;
@@ -129,8 +130,14 @@ EXPORT_SYMBOL_GPL(iio_hw_consumer_alloc);
 void iio_hw_consumer_free(struct iio_hw_consumer *hwc)
 {
 	struct hw_consumer_buffer *buf, *n;
+	struct iio_channel *chan = &hwc->channels[0];
+
+	while (chan->indio_dev) {
+		chan->indio_dev->mutex_class = IIO_MUTEX_NORMAL;
+		iio_channel_release(chan);
+		chan++;
+	}
 
-	iio_channel_release_all(hwc->channels);
 	list_for_each_entry_safe(buf, n, &hwc->buffers, head)
 		iio_buffer_put(&buf->buffer);
 	kfree(hwc);

drivers/iio/industrialio-buffer.c

@@ -1077,7 +1077,7 @@ int iio_update_buffers(struct iio_dev *indio_dev,
 		return 0;
 
 	mutex_lock(&indio_dev->info_exist_lock);
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock_nested(&indio_dev->mlock, indio_dev->mutex_class);
 
 	if (insert_buffer && iio_buffer_is_active(insert_buffer))
 		insert_buffer = NULL;

drivers/iio/industrialio-core.c

@@ -1454,6 +1454,7 @@ struct iio_dev *iio_device_alloc(int sizeof_priv)
 		dev->dev.groups = dev->groups;
 		dev->dev.type = &iio_device_type;
 		dev->dev.bus = &iio_bus_type;
+		dev->mutex_class = IIO_MUTEX_NORMAL;
 		device_initialize(&dev->dev);
 		dev_set_drvdata(&dev->dev, (void *)dev);
 		mutex_init(&dev->mlock);
@@ -1805,7 +1806,7 @@ EXPORT_SYMBOL_GPL(devm_iio_device_unregister);
  */
 int iio_device_claim_direct_mode(struct iio_dev *indio_dev)
 {
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock_nested(&indio_dev->mlock, indio_dev->mutex_class);
 
 	if (iio_buffer_enabled(indio_dev)) {
 		mutex_unlock(&indio_dev->mlock);

include/linux/iio/iio.h

@@ -17,6 +17,11 @@
  * Currently assumes nano seconds.
  */
 
+enum iio_mutex_lock_class {
+	IIO_MUTEX_NORMAL,
+	IIO_MUTEX_HWC,
+};
+
 enum iio_shared_by {
 	IIO_SEPARATE,
 	IIO_SHARED_BY_TYPE,
@@ -537,6 +542,7 @@ struct iio_dev {
 	struct list_head		buffer_list;
 	int				scan_bytes;
 	struct mutex			mlock;
+	int				mutex_class;
 
 	const unsigned long		*available_scan_masks;
 	unsigned			masklength;