iio: fix sched WARNING "do not call blocking ops when !TASK_RUNNING"

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@chromium.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org> # 3.19+ for introduction of wake_woken
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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;
@@ -131,19 +132,29 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
 	else
 		to_wait = min_t(size_t, n / datum_size, rb->watermark);
 
+	add_wait_queue(&rb->pollq, &wait);
 	do {
-		ret = wait_event_interruptible(rb->pollq,
-		      iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size));
-		if (ret)
-			return ret;
+		if (!indio_dev->info) {
+			ret = -ENODEV;
+			break;
+		}
 
-		if (!indio_dev->info)
-			return -ENODEV;
+		if (!iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size)) {
+			if (signal_pending(current)) {
+				ret = -ERESTARTSYS;
+				break;
+			}
+
+			wait_woken(&wait, TASK_INTERRUPTIBLE,
+				   MAX_SCHEDULE_TIMEOUT);
+			continue;
+		}
 
 		ret = rb->access->read_first_n(rb, n, buf);
 		if (ret == 0 && (filp->f_flags & O_NONBLOCK))
 			ret = -EAGAIN;
 	 } while (ret == 0);
+	remove_wait_queue(&rb->pollq, &wait);
 
 	return ret;
 }