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

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Date:	Thu, 4 Aug 2016 10:45:39 +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 10:26 AM, Brian Norris wrote:
> When using CONFIG_DEBUG_ATOMIC_SLEEP, the scheduler nicely points out
> that we're calling sleeping primitives within the wait_event loop, which
> means we might clobber the task state:
> 
> [   10.831289] do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffc00026b610>]
> [   10.845531] ------------[ cut here ]------------
> [   10.850161] WARNING: at kernel/sched/core.c:7630
> ...
> [   12.164333] ---[ end trace 45409966a9a76438 ]---
> [   12.168942] Call trace:
> [   12.171391] [<ffffffc00024ed44>] __might_sleep+0x64/0x90
> [   12.176699] [<ffffffc000954774>] mutex_lock_nested+0x50/0x3fc
> [   12.182440] [<ffffffc0007b9424>] iio_kfifo_buf_data_available+0x28/0x4c
> [   12.189043] [<ffffffc0007b76ac>] iio_buffer_ready+0x60/0xe0
> [   12.194608] [<ffffffc0007b7834>] iio_buffer_read_first_n_outer+0x108/0x1a8
> [   12.201474] [<ffffffc000370d48>] __vfs_read+0x58/0x114
> [   12.206606] [<ffffffc000371740>] vfs_read+0x94/0x118
> [   12.211564] [<ffffffc0003720f8>] SyS_read+0x64/0xb4
> [   12.216436] [<ffffffc000203cb4>] el0_svc_naked+0x24/0x28
> 
> To avoid this, we should (a la https://lwn.net/Articles/628628/) use the
> wait_woken() function, which avoids the nested sleeping while still
> handling races between waiting / wake-events.
> 
> Signed-off-by: Brian Norris <briannorris@...omium.org>

Thanks for taking care of this. Looks good, just one thing.

>  drivers/iio/industrialio-buffer.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index 90462fcf5436..2ad10e0190d8 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -107,6 +107,7 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>  {
>  	struct iio_dev *indio_dev = filp->private_data;
>  	struct iio_buffer *rb = indio_dev->buffer;
> +	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>  	size_t datum_size;
>  	size_t to_wait;
>  	int ret;
> @@ -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()

And as a minor improvement I'd also move the
add_wait_queue()/remove_wait_queue() outside of the outer loop. And then
just if (!iio_buffer_ready(...)) continue; rather than having the inner
loop. This should slightly simplify the flow. Just make sure to replace the
returns in the loop with a break so remove_wait_queue() has a chance to run.


> +		}
> +		remove_wait_queue(&rb->pollq, &wait);
>  
>  		if (!indio_dev->info)
>  			return -ENODEV;
>