Subject: Re: [PATCH][RFC] iio: core: add a class hierarchy on iio device lock

From: Olivier MOYSAN <[email-masked]>
Date: Mon, 14 Oct 2019 15:59:53 +0000

Hello Jonathan,

Thanks for your comment.

On 10/12/19 10:57 AM, Jonathan Cameron wrote:
> On Fri, 11 Oct 2019 17:13:14 +0200
> Olivier Moysan <[email-masked]> wrote:
>
>> 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
> Hmm.  I'm not sure I follow the logic.  That lock is
> for one thing and one thing only, preventing access
> to the iio device that are unsafe when it is running
> in a buffered mode.  We shouldn't be in a position where
> we both say don't do this if we are in buffered mode, + enter
> buffered mode whilst doing this, or we need special functions
> for entering buffering mode if in this state.  We are in
> some sense combining internal driver logic with overall
> IIO states.  IIO shouldn't care that the device is using
> the same methods under the hood for buffered and non
> buffered operations.
>
> I can't really recall how this driver works.   Is it actually
> possible to have multiple hw_consumers at the same time?
>
> So do we end up with multiple buffers registered and have
> to demux out to the read_raw + the actual buffered path?
> Given we have a bit of code saying grab one sample, I'm
> going to guess we don't...
>
> If so, the vast majority of the buffer setup code in IIO
> is irrelevant here and we just need to call a few of
> the callbacks from this driver directly... (I think
> though I haven't chased through every corner.
>
> I'd rather avoid introducing this nesting for a corner
> case that makes no 'semantic' sense in IIO as it leaves us
> in two separate states at the same time that the driver
> is trying to make mutually exclusive.  We can't both
> not be in buffered mode, and in buffered mode.
>
> Thanks and good luck with this nasty corner!
>
> Jonathan
>
Here I consider the following use case:
A single conversion is performed. The dfsdm (filter) is chained with a 
front-end, which can be an ADC or a sensor. So we have two IIO devices, 
the dfsdm and its front-end handled through the hw consumer interface.

You are right. There is something wrong here, in buffered/non-buffered 
mode mixing.
iio_hw_consumer_enable() call is used to enable the front-end device. 
But this interface is intended for buffered mode.
So this is not coherent with the expected single conversion mode, 
indeed. Another interface is required to manage the front-end device. I 
have a poor knowledge of iio framework, but it seems to me that there is 
no interface to manage this.

My understanding regarding mlock, is that it is used to protect the 
state of the iio device.
I we want to do a conversion from the chained devices, I think we need 
to activate the first device
and keep it performing conversion, as long as the second device has done 
its conversion.
We need to protect both devices, and we should have to do it in a nested 
way.
So, I guess that anyway, nested mutexes would be required in this case.

Best regards

Olivier

>
>> 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 #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 #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 <[email-masked]>
>> ---
>>   drivers/iio/buffer/industrialio-hw-consumer.c | 9 ++++++++-
>>   drivers/iio/industrialio-buffer.c             | 2 +-
>>   drivers/iio/industrialio-core.c               | 3 ++-
>>   include/linux/iio/iio.h                       | 6 ++++++
>>   4 files changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/iio/buffer/industrialio-hw-consumer.c b/drivers/iio/buffer/industrialio-hw-consumer.c
>> index 95165697d8ae..652ce31b4b5f 100644
>> --- a/drivers/iio/buffer/industrialio-hw-consumer.c
>> +++ b/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);
>> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
>> index c193d64e5217..d1df04167978 100644
>> --- a/drivers/iio/industrialio-buffer.c
>> +++ b/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;
>> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
>> index f72c2dc5f703..b14ba42559a3 100644
>> --- a/drivers/iio/industrialio-core.c
>> +++ b/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);
>> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
>> index 862ce0019eba..1192eca124f4 100644
>> --- a/include/linux/iio/iio.h
>> +++ b/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;
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