Patch "iio: fix sched WARNING "do not call blocking ops when !TASK_RUNNING"" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
iio-fix-sched-warning-do-not-call-blocking-ops-when-task_running.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Brian Norris <[email protected]>
Date: Mon, 8 Aug 2016 17:19:38 -0700
Subject: iio: fix sched WARNING "do not call blocking ops when !TASK_RUNNING"

From: Brian Norris <[email protected]>

commit fcf68f3c0bb2a541aa47a2a38b8939edf84fd529 upstream.

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.

---
 drivers/iio/industrialio-buffer.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

@@ -107,6 +107,7 @@ ssize_t iio_buffer_read_first_n_outer(st
 {
 	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(st
 	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)
-			return -ENODEV;
+		if (!indio_dev->info) {
+			ret = -ENODEV;
+			break;
+		}
+
+		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;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/i2c-cros-ec-tunnel-fix-usage-of-cros_ec_cmd_xfer.patch queue-4.4/iio-fix-sched-warning-do-not-call-blocking-ops-when-task_running.patch queue-4.4/mfd-cros_ec-add-cros_ec_cmd_xfer_status-helper.patch