[RFC PATCH] iio: trigger: Fix a scheduling whilst atomic issue seen on tsc2046

[Jonathan Cameron <jic23@kernel.org>]

From: Jonathan Cameron <Jonathan.Cameron@huawei.com>

IIO triggers are software IRQ chips that split an incoming IRQ into
separate IRQs routed to all devices using the trigger.
When all consumers are done then a trigger callback reenable() is
called.  There are a few circumstances under which this can happen
in atomic context.

1) A single user of the trigger that calls the iio_trigger_done()
function from interrupt context.
2) A race between disconnecting the last device from a trigger and
the trigger itself sucessfully being disabled.

To avoid a resulting scheduling whilst atomic, close this corner
by using schedule_work() to ensure the reenable is not done in atomic
context.

Note that drivers must be careful to manage the interaction of
set_state() and reenable() callbacks to ensure appropriate reference
counting if they are relying on the same hardware controls.

Completely untested beyond building. Given it is Sunday evening
very high chance of it being entirely wrong.  The potential race
conditions around this were giving me a headache, but I think this
is at least a step in the right direction and should work for Oleksij's
case.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
 drivers/iio/industrialio-trigger.c | 36 +++++++++++++++++++++++++++++-
 include/linux/iio/trigger.h        |  2 ++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
index b23caa2f2aa1..d3bdc9800b4a 100644
--- a/drivers/iio/industrialio-trigger.c
+++ b/drivers/iio/industrialio-trigger.c
@@ -162,6 +162,39 @@ static struct iio_trigger *iio_trigger_acquire_by_name(const char *name)
 	return trig;
 }
 
+static void iio_reenable_work_fn(struct work_struct *work)
+{
+	struct iio_trigger *trig = container_of(work, struct iio_trigger,
+						reenable_work);
+
+	/*
+	 * This 'might' occur after the trigger state is set to disabled -
+	 * in that case the driver should skip reenabling.
+	 */
+	trig->ops->reenable(trig);
+}
+
+/*
+ * In general, reenable callbacks may need to sleep and this path is
+ * not performance sensitive, so just queue up a work item
+ * to reneable the trigger for us.
+ *
+ * Races that can cause this.
+ * 1) A handler occurs entirely in interrupt context so the counter
+ *    the final decrement is still in this interrupt.
+ * 2) The trigger has been removed, but one last interrupt gets through.
+ *
+ * For (1) we must call reenable, but not in atomic context.
+ * For (2) it should be safe to call reenanble, if drivers never blindly
+ * reenable after state is off.
+ */
+static void iio_trigger_notify_done_atomic(struct iio_trigger *trig)
+{
+	if (atomic_dec_and_test(&trig->use_count) && trig->ops &&
+	    trig->ops->reenable)
+		schedule_work(&trig->reenable_work);
+}
+
 void iio_trigger_poll(struct iio_trigger *trig)
 {
 	int i;
@@ -173,7 +206,7 @@ void iio_trigger_poll(struct iio_trigger *trig)
 			if (trig->subirqs[i].enabled)
 				generic_handle_irq(trig->subirq_base + i);
 			else
-				iio_trigger_notify_done(trig);
+				iio_trigger_notify_done_atomic(trig);
 		}
 	}
 }
@@ -535,6 +568,7 @@ struct iio_trigger *viio_trigger_alloc(struct device *parent,
 	trig->dev.type = &iio_trig_type;
 	trig->dev.bus = &iio_bus_type;
 	device_initialize(&trig->dev);
+	INIT_WORK(&trig->reenable_work, iio_reenable_work_fn);
 
 	mutex_init(&trig->pool_lock);
 	trig->subirq_base = irq_alloc_descs(-1, 0,
diff --git a/include/linux/iio/trigger.h b/include/linux/iio/trigger.h
index 096f68dd2e0c..4c69b144677b 100644
--- a/include/linux/iio/trigger.h
+++ b/include/linux/iio/trigger.h
@@ -55,6 +55,7 @@ struct iio_trigger_ops {
  * @attached_own_device:[INTERN] if we are using our own device as trigger,
  *			i.e. if we registered a poll function to the same
  *			device as the one providing the trigger.
+ * @reenable_work:	[INTERN] work item used to ensure reenable can sleep.
  **/
 struct iio_trigger {
 	const struct iio_trigger_ops	*ops;
@@ -74,6 +75,7 @@ struct iio_trigger {
 	unsigned long pool[BITS_TO_LONGS(CONFIG_IIO_CONSUMERS_PER_TRIGGER)];
 	struct mutex			pool_lock;
 	bool				attached_own_device;
+	struct work_struct		reenable_work;
 };
 
 
-- 
2.33.1