linux-kernel - Re: [PATCH] iio: fix sched WARNING "do not call blocking ops when !TASK

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Date:	Thu, 4 Aug 2016 12:21:08 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Brian Norris <briannorris@...omium.org>
Cc:	Jonathan Cameron <jic23@...nel.org>,
	Hartmut Knaack <knaack.h@....de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Guenter Roeck <linux@...ck-us.net>,
	Brian Norris <computersforpeace@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] iio: fix sched WARNING "do not call blocking ops when
 !TASK_RUNNING"

On 08/04/2016 11:41 AM, Brian Norris wrote:
> On Thu, Aug 04, 2016 at 10:45:39AM +0200, Lars-Peter Clausen wrote:
>>> @@ -132,10 +133,13 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>>>  		to_wait = min_t(size_t, n / datum_size, rb->watermark);
>>>  
>>>  	do {
>>> -		ret = wait_event_interruptible(rb->pollq,
>>> -		      iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size));
>>> -		if (ret)
>>> -			return ret;
>>> +		add_wait_queue(&rb->pollq, &wait);
>>> +		while (!iio_buffer_ready(indio_dev, rb, to_wait,
>>> +					 n / datum_size)) {
>>> +			wait_woken(&wait, TASK_INTERRUPTIBLE,
>>> +				   MAX_SCHEDULE_TIMEOUT);
>>
>> We loose the ability to break out from this loop by sending a signal to the
>> task. This needs something like
>>
>> 	if (signal_pending(current)) {
>> 		ret = -ERESTARTSYS;
>> 		break;
>> 	}
>>
>> before the wait_woken()
> 
> Sounds good.
> 
>> And as a minor improvement I'd also move the
>> add_wait_queue()/remove_wait_queue() outside of the outer loop.
> 
> Sure.
> 
>> And then
>> just if (!iio_buffer_ready(...)) continue; rather than having the inner
>> loop. This should slightly simplify the flow.
> 
> Perhaps I'm not gathering your meaning here, but wouldn't that turn this
> into a spin loop, waiting for iio_buffer_ready()? i.e.:
> 
> 	do {
> 		if (!iio_buffer_ready(...))
> 			continue; // we shouldn't just hammer
> 				  // iio_buffer_ready(), should we?
> 
> 		wait_woken(...);
> 		...
> 	};

Hm, right, I didn't think this through.

How about:

 	do {
		if (!indio_dev->info)
			return -ENODEV;

 		if (!iio_buffer_ready(...)) {
			if (signal_pending(current)) {
				ret = -ERESTARTSYS;
				break;
			}	
	 		wait_woken(...);
			continue;
		}
 		...
 	} while (ret == 0);

And then also drop the if (!indio_dev->info) at the beginning of the function.